Primary HD not booting after re-mirror

The primary (boot) HD on a mirrored NT 4.0 server crashed. The secondary HD took over, booted and kept working. When the primary HD was replaced and was re-mirrored, it would not boot. System never started to load NT, just hung. Had to bypass HD in SCSI BIOS in order for system to boot.
About to try re-loading NT on primary HD to start over. But, why would it not work? I did this before and it worked fine.