Spanned volumes combine areas of unallocated space from multiple disks into one logical volume. The areas of unallocated space can be different sizes. Spanned volumes require two disks, and you can use up to 32 disks. When creating spanned volumes, keep these points in mind:You can extend only NTFS volumes or unformatted volumes.After you create or extend a spanned volume, you cannot delete any portion of it without deleting the entire spanned volume.You cannot stripe or mirror spanned volumes. For more information about striped or mirrored volumes, see “Striped Volumes” or “Mirrored Volumes” later in this section.Spanned volumes do not provide fault tolerance. If one of the disks containing a spanned volume fails, the entire volume fails, and all data on the spanned volume becomes inaccessible. The reliability for a spanned volume is less than the least reliable disk in the set.
From Microsoft Technet
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