All four SATA drives had somehow become reconfigured as IDE, so I changed them back to RAID. On boot, the RAID status screen appears and shows the two RAID arrays as normal, but then an error message appears:
Is there a way to fix this?
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I re-configured the drives as RAID instead of IDE, although I have no idea how they were changed to IDE in the first place.
Then I disconnected one drive, then re-connected it. When the computer started up again, the RAID display showed the disconnected drive as "degraded", and when Windows came up I checked the Intel RAID manager and found that the drives were being rebuilt.
After successful rebuilds of both mirrors, the system is working just fine.
I just hope that whatever happened to break the RAID mirrors doesn't happen again.