Intel Software Raid: Downgrade from RAID10 to RAID 1

My configuration

• Windows 8.1
• Asus Z87I Pro (Rev C2)
• 4x WD Red 3TB, set up as RAID10 in the Intel tool / EFI

My target

• I want to remove 2 of the HDDs permanently from the RAID because I don’t need that much space anymore. The Raid should be RAID 1 afterwards (mirroring)
• None of the discs is faulty.
• I don’t want to re-install Windows 8.1, obviously.

My question

• What do I have to do to downgrade this RAID10 to RAID1, such that I will be able to remove 2 of the 4 HDDs afterwards?

You aren’t going to be able to just “switch” RAID modes like that, as the array would need to be recreated/reinitialized.

Easiest:

• Reduce data so it’s small enough to fit on the RAID1.
• Get an external drive that’s large enough to accommodate your data (and then some).
• Perform full system backup to external drive.
• Destroy RAID10.
• Remove 2 drives.
• Create RAID1.
• Restore Backup from external.

If you can’t obtain an external drive (aka: “slow, nitpicky, dangerous way”):

• Reduce data so it’s small enough to fit on the RAID1.
• Determine which drives (by serial number, etc.) are in which part of the RAID10, and pull the two you can afford to loose (that won’t bring the array down).
• Create an additional array that’s RAID1, using the two drives you detached from the RAID10.
• Perform full system backup from degraded RAID10 to healthy RAID1.
• Destroy RAID10.
• Create RAID1 with two drives that were last in RAID10.
• Restore Backup from first RAID1 you made.
• Destroy RAID1 you first made.
• Remove 2 drives.

Note: Have RAID controller drivers available on USB at all times.