When the master drive on the primary channel of a mirrored Windows NT drive fails, the system times out and appears to hang.
This will happen even if the shadow drive is configured as the master on the secondary EIDE controller.
You will also experience erratic mouse movement during the timeout period (which varies based on hardware and the processes being run at the point of failure).
This is normal failover behavior for a mirrored IDE drive (SCSI based drives failover immediately).
When the system or boot drive fails, you will have to use a Boot Floppy to boot from the shadow, or you will have to reconfigure the shadow as the master drive on the primary controller.
If the shadowed drives have different drive geometry, you may still be unable to boot.
See the following Knowledge Base articles:
Q141702 - How to Recover Mirroring Windows NT Using IDE devices.
Q114779 - Overview of Disk Mirroring (RAID Level 1) in Windows NT.
Q128630 - How to Recover From a STOP 0x00000058 FTDISK_INTERNAL_ERROR.