Powered down, re-connected the replacement drive, and booted up. It showed the replacement as non-RAID. I used the RAID BIOS to add it to the mirrored volume, and Windows booted up, marking the drive off-line. Click on the drive in ISM, used the wizard to restore the array to the new disk, and things were fine. For about half an hour. At about 6% ISM marked the drive as failed.
So I repeated the process with a second replacement drive, same result. That was last night.
This morning I reconnected the original replacement, and let the machine boot with it as non-RAID. It came up as drive D, which I formatted. Then I opened ISM and told it to restore the array to that disk, and so far it's at 22% done. I really hope it completes, but won't know till after I get back from dim sum and a movie this afternoon.
Either way, I'm hooking up the firewire backup drive and making a fresh backup.
Edit add: Nope, it failed again. I'm thinking it's the SATA cable. I'm on the PC right now, backup will take about 3.5 hours more (8 hours total)