Mirrored RAID problem--can't see drive 2

I'm a novice with RAIDs. I was testing a mirrored RAID to make sure both drives would boot on their own--which they did (hit ESC). When I connected both drives back, however, the computer can't see the second drive. Even after booting into Windows, it d/n see the drive. I can still boot off the second drive, but it d/n appear anywhere else. This tells me the drive is physically good. This makes no sense. What gives? This is a Promise Tech RAID.