RAID Recovery™
Recovers all types of corrupted RAID arrays
Recovers all types of corrupted RAID arrays
Last updated: Sep 27, 2023

How to Rebuild RAID Without Losing Data

RAID (Redundant Array of Independent Disks) has become a mainstay in data storage configurations, prized for its ability to combine multiple drives into one unified system, ensuring both performance and redundancy. Yet, like all technologies, RAID isn't infallible. Whether due to disk failures, accidental changes, or other unforeseen issues, there might come a time when your RAID setup requires rebuilding. The fear of potential data loss during such times can be overwhelming. But fret not. This article aims to demystify the RAID rebuilding process, guiding you through each step to ensure that your data remains secure and intact. Whether you're a seasoned IT professional or a RAID rookie, by the end of this guide, you'll have a comprehensive understanding of how to tackle RAID challenges head-on. Let's embark on this journey of data recovery and RAID restoration together!

Overview Of The RAID Levels

RAID (Redundant Array of Independent Disks) is a storage technology that combines multiple hard drives into a single logical unit for the purposes of data redundancy, improved performance, or both. Different RAID configurations, or levels, have been developed to address various needs. Here's an overview of the most common RAID levels:

  1. 1. RAID 0 (Striping):

    • Description: Data is split across all drives without redundancy. It enhances performance but offers no fault tolerance.
    • Advantage: High performance due to increased throughput.
    • Disadvantage: If one drive fails, all data is lost.
    • Use Cases: Situations where performance is a priority over data redundancy, such as in gaming or video editing setups.
  2. 2. RAID 1 (Mirroring):

    • Description: Data is duplicated across two or more drives, ensuring that a copy exists on each.
    • Advantage: High fault tolerance. If one drive fails, no data is lost.
    • Disadvantage: Costs more due to duplicated storage needs; half of total storage is usable.
    • Use Cases: Critical data storage where data loss cannot be afforded.
  3. 3. RAID 5 (Striping with Parity):

    • Description: Data is striped across drives, similar to RAID 0, but with the addition of parity information which allows data recovery in the event of a single drive failure.
    • Advantage: Combines good fault tolerance with efficient storage use.
    • Disadvantage: Write performance may be slower due to parity calculations.
    • Use Cases: Servers and performance-critical setups where some redundancy is required.
  4. 4. RAID 6 (Striping with Double Parity):

    • Description: Similar to RAID 5 but includes two parity blocks instead of one. This allows the array to recover from the failure of two drives simultaneously.
    • Advantage: Improved fault tolerance over RAID 5.
    • Disadvantage: Write performance hit and reduced storage efficiency compared to RAID 5.
    • Use Cases: Critical data storage setups where the potential concurrent failure of two drives is a concern.
  5. 5. RAID 10 (or 1+0):

    • Description: A combination of RAID 1 and RAID 0. It mirrors data and then stripes it across multiple mirrored sets.
    • Advantage: High fault tolerance and performance.
    • Disadvantage: Only 50% of total drive capacity is usable.
    • Use Cases: Databases and other high-performance applications requiring redundancy.
  6. 6. RAID 50 (5+0):

    • Description: Combines the features of RAID 5 and RAID 0. It stripes data across RAID 5 sets.
    • Advantage: Offers a balance between performance, data protection, and usable capacity.
    • Disadvantage: Requires a minimum of 6 drives and can be complex to set up.
    • Use Cases: Large databases and mission-critical applications.

There are other RAID configurations and many nuances within each level, but this overview covers the most common ones. When choosing a RAID level, it's essential to consider your specific needs concerning performance, redundancy, and cost.

What Is RAID Rebuilding?

RAID Rebuilding refers to the process of restoring data to a RAID array after the replacement of a failed or malfunctioning disk. This process is crucial in RAID configurations that offer redundancy, such as RAID 1, RAID 5, RAID 6, and RAID 10, among others.

When one or more disks in a RAID setup fail but not enough to cause a total system failure (thanks to the redundancy built into certain RAID levels), the RAID system can continue functioning in a degraded mode. This means that while all the data is still accessible, the system is at risk because the redundancy is compromised. To bring the system back to a fully redundant state, the failed disk needs to be replaced, and the data that was on the failed disk must be rebuilt onto the new one.

Here's a breakdown of the RAID rebuilding process:

  1. 1. Detection of Failure: The RAID controller or software detects that a drive has failed. This is often accompanied by an alert or notification.

  2. 2. Drive Replacement: The failed drive is physically removed from the system and replaced with a new, functioning one.

  3. 3. Rebuild Initiation: Once the new drive is installed and recognized, the RAID system starts the rebuilding process. Depending on the RAID setup and management software, this might start automatically or may require manual initiation.

  4. 4. Data Restoration: Using the redundancy data (like parity in RAID 5 or mirror in RAID 1), the system reconstructs the missing data onto the new drive.

  5. 5. Completion: Once the rebuilding process is complete, the RAID array returns to its full operational and redundant state.

It's worth noting a few important points about RAID rebuilding:

  • Duration: The rebuilding process can take a significant amount of time, especially for large drives or in configurations with large amounts of data. The time taken depends on factors like drive size, RAID level, system load, and others.
  • System Load: While RAID systems can continue to operate during a rebuild, performance might be reduced due to the added strain of the rebuilding process.
  • Risk: The RAID array is more vulnerable during the rebuild process. If another drive fails before the rebuild completes (in setups like RAID 5 where only one drive failure is tolerated), data loss can occur.

Given the complexities and risks, it's essential to monitor RAID systems regularly, ensure that backups are current, and replace failed drives as soon as possible.

How to rebuild RAID array?

Step 1: Prepare the Array

Before diving directly into the rebuilding process, it's crucial to prepare the RAID array to ensure a smooth and successful rebuild. Here's a systematic guide to help you prepare your RAID array for rebuilding:

  1. 1. Backup Your Data:

    • Before making any changes, always back up your data. Even though RAID provides redundancy, it's not a substitute for a proper backup. Ensure you have a complete and up-to-date backup of all essential data stored on the RAID array.
  2. 2. Check System Health:

    • Use RAID management software or RAID controller BIOS to review the status of all drives in the array. Identify any other potential issues that might not yet have triggered a failure state but could complicate the rebuilding process.
    • Monitor the temperature and overall health of the system to ensure that it's running within optimal ranges.
  3. 3. Identify the Failed Drive:

    • Using your RAID management tools, determine which drive has failed. The software should indicate the status of each drive, making it easy to identify the problematic one.
    • Make a note of the failed drive's position and serial number, if possible, to avoid confusion during replacement.
  4. 4. Acquire a Suitable Replacement Drive:

    • Ensure the replacement drive is of equal or larger capacity than the failed drive. Ideally, it should be of the same make and model to ensure compatibility.
    • Check if the replacement drive has no existing RAID metadata or partitions. If it was previously part of another RAID array or used in another system, you might need to clear its configuration.
  5. 5. Prepare RAID Management Tools:

    • Ensure you have access to the RAID management software or tools specific to your RAID controller or motherboard. You'll need these tools to monitor the rebuild process and possibly to initiate the rebuild, depending on your RAID setup.
    • Update the RAID controller firmware or software if necessary. However, be cautious; ensure backups are secured before making such updates.
  6. 6. Schedule Downtime (If Necessary):

    • Depending on your RAID setup and the criticality of the systems relying on it, you might need to schedule a period of reduced performance or even downtime. This ensures that the rebuild process has the system resources it needs and reduces strain on the remaining drives.

With the RAID array properly prepared, you can proceed confidently to the actual rebuilding step, ensuring that the risk of further issues or data loss is minimized.

Step 2: Connect the Replacement Drive to the Controller

After preparing the RAID array and securing a suitable replacement drive, the next step involves physically connecting this new drive to the RAID controller. This step is vital because the RAID controller will handle the data rebuilding onto the new drive.

  1. 1. Power Down (If Required):

    • Depending on your RAID setup and the design of your hardware, you may need to power down the system before replacing the drive, especially if you don't have a hot-swappable setup.
    • If your setup supports hot-swapping (which allows drives to be removed and replaced without shutting down the system), ensure you follow the appropriate procedures to safely remove the failed drive.
  2. 2. Remove the Failed Drive:

    • Carefully remove the failed drive from its bay or slot. It's essential to ensure you're removing the correct drive; refer to the notes or indicators from your RAID management tools as mentioned in the preparation step.
    • Handle the drives with care to avoid any physical damage or static discharge. It's advisable to wear an anti-static wrist strap to protect sensitive electronics.
  3. 3. Install the Replacement Drive:

    • Slide the replacement drive into the now-empty bay or slot where the failed drive was located.
    • Ensure that the drive is properly seated and that all connections (data and power) are secure.
    • If your system was powered down, turn it back on. If you're using hot-swapping, the RAID controller should detect the new drive immediately.
  4. 5. Verify Drive Detection:

    • Once the system is powered on, access the RAID management software or RAID controller BIOS.
    • Check to ensure that the new drive is detected and is listed as an available drive. If it's not detected, double-check the connections.
  5. 5. Clear Old Metadata (If Necessary):

    • If the replacement drive was previously part of another RAID setup or had data, it might have old metadata. Before it can be used in the rebuild, this old metadata typically needs to be cleared.
    • Using the RAID management software, there should be an option to clear or reset the drive, preparing it for integration into your RAID array.
  6. 6. Set as a Replacement (If Required):

    • Some RAID management tools might require you to manually designate the new drive as a replacement for the failed one. In others, the system might automatically start the rebuild process.
    • Follow the software's instructions or prompts to set the new drive as the replacement.

With the replacement drive successfully connected to the RAID controller and recognized by the RAID management tools, you're now set to initiate the rebuilding process, restoring redundancy and full functionality to your RAID array.

Step 3: Recover Array Parameters

Once the new drive is connected to the RAID controller, the next step is to ensure that the RAID system recognizes the new drive as part of the original RAID configuration and starts rebuilding the data onto it. Recovering the array parameters helps the RAID system restore and replicate the data structure and layout from the other drives in the array onto the new drive.

  1. 1. Access RAID Management Tools:

    • Boot into your RAID controller's BIOS or use the RAID management software provided by your RAID card manufacturer or motherboard vendor.
  2. 2. Verify Drive Status:

    • The new drive should be listed among the available drives, and its status might be indicated as 'Unconfigured', 'Spare', or similar, depending on the software.
    • Ensure that the other drives in the array show their status as 'Online', 'Healthy', or similar. This indicates that they are functioning correctly.
  3. 3. Recover Array Configuration:

    • Most modern RAID controllers automatically detect the loss of a drive and the addition of a replacement. If the array parameters were lost or altered (for instance, if the RAID configuration was accidentally reset), some RAID management tools have an option to "Recover RAID Configuration" or "Import Foreign Configuration".
    • By selecting this option, the RAID controller will scan the drives for existing RAID metadata and attempt to recover the original RAID configuration without altering the data.
  4. 4. Assign Replacement Drive (If Necessary):

    • If the RAID controller doesn't automatically begin the rebuild process, you might need to manually assign the new drive to the array.
    • Using the management tools, assign the new drive as a replacement or hot spare for the RAID array in question. Depending on the software, this might involve right-clicking on the drive, selecting it from a list, or navigating through a menu.
  5. 5. Initiate Manual Rebuild (If Required):

    • Some RAID systems will automatically start the rebuild process once the new drive is recognized and assigned. Others might require you to manually initiate the rebuild.
    • Locate the option in the RAID management software to start the rebuild, often labeled "Start Rebuild", "Rebuild Array", or similar.
  6. 6. Monitor Rebuild Progress:

    • The RAID management software should provide a way to monitor the progress of the rebuild. This can be crucial as rebuilding, especially on large drives or complex RAID configurations, can take a considerable amount of time.
    • Keep an eye on the status of the other drives in the array during this process to ensure they remain healthy.

It's essential to understand that during the rebuilding process, the RAID array might operate at a degraded performance level. It's recommended not to subject the system to heavy workloads during this period to ensure a smooth and error-free rebuild.

With the array parameters recovered and the rebuild process initiated, your RAID system should soon return to its optimal, redundant state, ensuring the security and accessibility of your data.

Step 4: Rebuild the New Drive into the Array

After ensuring that the RAID system parameters are intact and the new drive is correctly recognized, the next crucial step is the actual rebuilding of data onto the new drive. This process involves the RAID controller using the existing data and redundancy information from the other drives to recreate the lost data on the replacement drive.

  1. 1. Automatic Rebuild:

    • Modern RAID controllers or software solutions often automatically initiate the rebuilding process once a new drive is detected and assigned as a replacement.
    • Ensure the RAID system is set to auto-rebuild if this is your preferred method. Some systems may have this feature turned off by default for various reasons.
  2. 2. Manual Rebuild Initiation:

    • If the system doesn't start the rebuild automatically or if you previously stopped an auto-rebuild for any reason, you'll need to manually start the process.
    • Within your RAID management software or BIOS interface, locate and select the option to rebuild the RAID array. This might be labeled as "Rebuild Array", "Start Rebuild", or something similar.
  3. 3. Monitor the Rebuild Process:

    • The RAID management interface should provide a progress bar or percentage indicator showing the progress of the rebuild. Monitor this closely.
    • Note that the rebuild process can take considerable time, especially for large volumes or if the system is under load. For vast arrays, it can take several hours to days.
  4. 4. Maintain Minimal System Load:

    • It's advisable to minimize intensive system operations during the rebuild. While the RAID system can handle regular operations during this time, high loads can slow down the rebuild process and put extra stress on the other drives.
  5. 5. Verify Successful Rebuild:

    • Once the rebuild process is complete, the RAID management tool should indicate that the RAID array is in a 'Healthy' or 'Optimal' state. The new drive should also show a status like 'Online' or 'Active'.
    • If any errors occurred during the rebuild, they should be logged in the RAID management software. Address these errors as necessary.
  6. 6. Backup and Test:

    • After the rebuild, it's a good practice to create a fresh backup of your data.
    • Test the RAID array by accessing data, creating new files, and performing typical operations to ensure everything is functioning as expected.
  7. 7. Maintenance and Monitoring:

    • Use this rebuild as an opportunity to set up or review regular monitoring for your RAID system. Regular monitoring helps in early detection of potential issues, ensuring data safety.
    • Periodically check the health of all drives in the array to prevent potential future failures.

A successfully rebuilt RAID array should restore the system's redundancy and data protection capabilities. However, remember that while RAID offers redundancy, it's not a substitute for regular backups. Always keep your backups updated and routinely test them to ensure data integrity and availability.

Step 5: Write Data to the New Array

By this stage, the RAID array is rebuilt and operational with the new drive. Now, it's time to utilize the restored array by writing new data, ensuring that the system functions as intended post-rebuild.

  1. 1. Verify Array Health:

    • Before adding new data, double-check the status of the RAID array. Ensure it's listed as 'Healthy', 'Optimal', or a similar positive status in your RAID management tool.
  2. 2. Update System or Application Settings (If Necessary):

    • If the RAID array hosts critical applications or databases, ensure that any configuration files or settings pointing to the array are correctly set up. Sometimes, after such operations, certain applications might require a nudge to recognize the restored environment.
  3. 3. Write Test Data:

    • Before moving critical data or resuming regular operations, consider writing some test data to the array. This can be a large file or a set of files.
    • After writing, try reading back the data and verify its integrity. This test ensures the RAID array is functioning correctly post-rebuild.
  4. 4. Resume Regular Operations:

    • If the test data writes and reads back successfully, you can be reasonably confident in the array's operational status.
    • Resume regular operations, whether that's saving new files, running applications, or whatever tasks your RAID system typically handles.
  5. 5. Monitor Performance:

    • Keep an eye on the RAID array's performance during the initial hours or days after the rebuild. Ensure data transfer speeds are as expected, and there's no unusual lag or delay.
    • Use system monitoring tools to watch for any unusual spikes in resource usage.
  6. 6. Document the Process:

    • Keeping records can be invaluable for future reference. Document the date of the RAID rebuild, the drive replaced, any issues encountered, and their resolutions.
  7. 7. Implement Data Integrity Checks:

    • Depending on the nature of your data and its importance, consider implementing periodic data integrity checks. Tools can validate that data hasn't been corrupted over time, which is especially vital if the RAID hosts critical or sensitive information.
  8. 8. Backup the New Data:

    • Any new data written to the RAID array should be included in your regular backup routines. If you've made significant changes or additions post-rebuild, consider performing an immediate backup.
  9. 9. Stay Updated and Prepared:

    • Ensure that your RAID controller's firmware and any associated software are updated to the latest versions. Manufacturers often release updates that fix known issues or enhance performance.
    • Consider keeping a spare drive on hand, especially for RAID configurations that can only tolerate a single drive failure. Being prepared can reduce downtime in case of another failure.

Successfully writing to and operating the newly rebuilt RAID array reaffirms the system's health and functionality. Regular monitoring, maintenance, and backups will continue to be your best allies in ensuring data security and system uptime.

Step 6. Copy Data Back

In scenarios where the RAID array was entirely wiped or the decision was made to start with a fresh RAID configuration, you'd need to restore or copy back your data from backups. This step is crucial, and it's imperative to handle it systematically to ensure that all data is restored without corruption.

  1. 1. Verify the Health and Status of the RAID Array:

    • Before copying any data back, ensure the RAID array is fully functional. The status should reflect as 'Healthy', 'Optimal', or a similar positive indication in the RAID management tool.
  2. 2. Prepare the Backup Source:

    • Connect or access your backup storage. Ensure that it's functioning correctly, and the data is accessible.
    • If you're using a backup software or service, launch it and ensure it's ready for the restoration process.
  3. 3. Plan the Data Restoration:

    • Determine the order in which data should be restored. For businesses, it might be crucial to restore mission-critical applications and databases first.
    • Ensure that you're familiar with the directory structure of the backup so you can effectively restore data to its proper location on the RAID array.
  4. 4. Begin the Data Restoration Process:

    • Using your backup software or manually (if it's a simple file-based backup), start the data copy or restoration process.
    • Monitor the process for any errors. If errors are encountered, note them for troubleshooting.
  5. 5. Validate the Data:

    • After the restoration process, validate the data. This means checking files, launching applications, and running databases to ensure everything operates as expected.
    • Check file sizes, date modified attributes, and, if possible, use checksum tools to verify data integrity.
  6. 6. Reconfigure Applications or Services (If Necessary):

    • Some applications or services may need reconfiguration post-restoration, especially if there were changes in the RAID setup or if IP addresses, drive letters, or paths have changed.
  7. 7. Update and Patch:

    • Once your data and applications are restored, ensure that everything is updated. This includes the operating system, software applications, and any other relevant components. Doing so will ensure that you're not restoring vulnerabilities along with your data.
  8. 8. Backup Strategy Re-evaluation:

    • Post restoration, it's an excellent time to re-evaluate your backup strategy. Ensure that backups are scheduled regularly, that they are comprehensive, and consider implementing a 3-2-1 backup strategy (3 total copies of your data, 2 of which are local but on different mediums, and 1 is offsite).
  9. 9. Regular Monitoring:

    • Especially after such an extensive operation, keep a close eye on the RAID array's performance and health. Monitor for any unusual behavior or errors, addressing any issues promptly.
  10. 10. Document Everything:

  • Keeping a record of events, issues faced, and resolutions can be invaluable. Document the RAID failure, the steps taken for recovery, data restoration, and any other relevant details.

Restoring data to a RAID array is a significant step, and its success ensures the continuation of operations with minimal data loss. Once the data is restored, remember to keep the RAID array in good health through regular monitoring, timely updates, and consistent backups.

Recovery by RAID Configuration Type

Recovering data from a RAID configuration is a critical operation, and the approach varies based on the specific RAID level. Each RAID level has its unique architecture and failure tolerances. Below is an overview of recovery strategies for the most commonly used RAID configurations:

RAID 0 (Striped Disk Array)

Configuration: Data is striped across all drives. No redundancy is provided.

Recovery Process:

  1. 1. If one drive fails in RAID 0, all data is lost.
  2. 2. Recovery is generally reliant on specialized software or professional data recovery services.
  3. 3. A byte-level copy of each remaining drive is usually made for the recovery process.
  4. 4. Data fragments are then pieced together based on known stripe sizes and patterns.

RAID 1 (Mirrored Disk Array)

Configuration: Data is mirrored across two drives.

Recovery Process:

  1. 1. If one drive fails, data can be directly accessed from the mirror drive.
  2. 2. It's advised to replace the failed drive and rebuild the mirror immediately.
  3. 3. If both drives fail, specialized software or professional recovery services might be required.

RAID 5 (Striped Disk Array with Parity)

Configuration: Data and parity are striped across three or more drives.

Recovery Process:

  1. 1. Can tolerate one drive failure. If one drive fails, the array can still operate in a degraded mode.
  2. 2. Replace the failed drive and the system will rebuild data using parity from other drives.
  3. 3. If two drives fail simultaneously, data recovery becomes complex and might require specialized services.

RAID 6 (Striped Disk Array with Dual Parity)

Configuration: Like RAID 5, but with dual parity, allowing it to handle the failure of two drives.

Recovery Process:

  1. 1. Can tolerate two simultaneous drive failures.
  2. 2. Replace the failed drives one at a time, allowing the array to rebuild after each insertion.
  3. 3. If more than two drives fail simultaneously, professional recovery services will likely be required.

RAID 10 (1+0, Mirrored and Striped)

Configuration: A combination of RAID 1 and RAID 0. It provides the redundancy of RAID 1 along with the striping of RAID 0.

Recovery Process:

  1. 1. Can tolerate multiple drive failures as long as they aren't from the same mirrored pair.
  2. 2. Replace the failed drive, and data will be rebuilt from its mirrored counterpart.
  3. 3. If both drives from a mirrored pair fail, recovery can become complex.

General Tips:

  • Regular Backups: No matter the RAID configuration, regular backups are essential. RAID is for redundancy and performance, not backup.
  • Monitor RAID Health: Use RAID management tools to keep an eye on drive health, ensuring timely interventions.
  • Avoid Stressing Degraded Arrays: If a RAID array is operating in a degraded state, avoid putting heavy loads on it until it's rebuilt.
  • Professional Services: If unsure about recovery, or if data is mission-critical, consider engaging professional RAID data recovery services.

It's essential to approach RAID recovery with care and caution. Mistakes can exacerbate data loss. Always prioritize data safety over speed of recovery.


  • Can I still access my data if one of the drives in my RAID setup fails?

    This depends on the RAID configuration. For instance:

    • RAID 0: No. If one drive fails, all data is lost because data is striped across all drives without redundancy.
    • RAID 1: Yes. If one drive fails, you can access data from the mirrored drive.
    • RAID 5: Yes, but in a degraded mode. RAID 5 can tolerate one drive failure by using parity data from the remaining drives.
    • Other RAID configurations will vary, so always refer to the specific RAID level's guidelines.
  • How long does it typically take to rebuild a RAID array?

    The rebuild time can vary based on factors like the size of the drives, the RAID configuration, the amount of data, the specific RAID controller, and the system's current workload. Generally, larger arrays or systems under heavy load take longer. It can range from a few hours to several days.

  • Is it safe to use my system while the RAID is rebuilding?

    While most RAID systems allow for continued use during a rebuild, it can slow down the rebuild process. It's also riskier since the system is already in a vulnerable state. If possible, it's best to minimize system use or run only light tasks until the rebuild completes.

  • I don't have a backup, and multiple drives failed in my RAID 5 setup. Can I still recover my data?

    RAID 5 can only tolerate one drive failure. If multiple drives fail, direct recovery via the RAID system isn't possible. However, you might have a chance with specialized RAID recovery software or professional data recovery services. It's crucial, though, not to make any changes or write new data to the drives, as it could further jeopardize recovery chances.

  • After replacing a failed drive, the RAID isn't rebuilding automatically. What should I do?

    Some RAID controllers or systems might not initiate an automatic rebuild for various reasons. In such cases:

    • Double-check that the replacement drive is properly connected and is recognized by the system.
    • Ensure the drive is of the correct size and type.
    • Manually initiate the rebuild process via the RAID management software or BIOS interface. Look for options like "Rebuild Array" or "Start Rebuild."

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