Simple volumes are the dynamic-disk equivalent of the primary partitions and logical drives found on basic disks. When creating simple volumes, keep these points in mind:
- By extending the simple volume on the same disk. The volume remains a simple volume, and you can still mirror it.
- By extending a simple volume to include unallocated space on other disks on the same computer. This creates a spanned volume.
If the simple volume is the system volume or the boot volume, you cannot extend it.
From Microsoft TechNet