About 6% into the rebuild, the disk manager software (Intel's special one for their onboard RAID) burped and said the new drive had failed. Not likely, since this was a drive I had taken out of a working array and it's been sitting on the shelf for a couple of months. We were working fine on the one alive boot drive, so I shut down the machine and rebooted to see what the RAID BIOS said about the drive. Usually it comes back up.
Not this time. This time it showed BOTH drives as "Offline Member Disk". WTF? Intel doesn't even have this phrase on their web site.
What's even more weird is the RAID chip is working - I have two other drives mirrored, and they come up as a good volume with two good member disks. But that's my data drive, and it is not bootable. So I'm SOL for the moment, waiting to hear back (Monday probably) from Intel tech support.