The minimum number of drives required for RAID 10 is four. RAID 10 disk drives are a combination of RAID 1 and RAID 0, the first step of which is to create a number of RAID 1 volumes by mirroring two drives together (RAID 1). The second step involves creating a stripe set with these mirrored pairs (RAID 0). In its original form this was called RAID 1+0, but has been abbreviated to RAID 10. If you had any less than four drives you would only be able to make a single RAID 1 mirror set.
RAID 10 should not be confused with RAID 0+1, which involves creating two RAID 0 stripe sets and then mirroring them. They have very similar characteristics and use the same number of drives, but are not the same.
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