Raid 1 Missing Hard Drive ??

[idata]

Had a Raid 1 drive fail. Replaced it with new same. Hit CTRL-I during reboot to enter config util and followed the prompts to add the new drive and initiate the auto re-build. Ok. Seems straightforward. Boot to Vista 64 and the rebuild begins. Enter IMSC and it shows the array rebuilding but it also shows that now it contains 3 drives and one of those drives is missing!!! Huh? How did that happen? More importantly, how do I correct it without it crashing the array?

[idata]

Strangely, after completing the rebuild the array as reported in IMSC appears to be normal and back down to just two drives. Peculiar.

[idata]

It may seem strange, but what you saw was the expected behavior. You started out with 2 drives in a RAID1 configuration. Let's call them drives A and B. Then Drive B failed. You removed Drive B, and inserted Drive C, and flagged it for a rebuild. When you removed drive B, you effectively had a RAID 1 volume with a missing disk that was rebuilding to a new disk C. The RAID controller will not "forget" about drive B until it has fully rebuilt to drive C. Only once drive C is fully operational will the RAID controller consider drive B to be replaced.