Before establishing a mirror, I strongly suggest that you upgrade to the most recent Service Pack, but at least SP4.
To guarantee that you can boot to the mirror drive, if the primary fails, create a Boot Floopy, and modify the boot.ini to have the ARC Path of the mirror drive as a boot option.
In order to be able to boot the shadow drive, without a Boot Floppy:
1. Both drives must be attached to identical controller, possibly with identical firmware.
2. Both drives must be identical, and may even require identical firmware.
3. The BIOS parameter block of the MBR must be identical on both drives.
4. Partitioning, up to the shadowed partition, must be identical on both drives.
5. If an EISA partition exists on the primary drive, it must be identical on the shadow.
6. The shadow must contain a valid boot.ini, NTLDR, and NTDETECT.COM, and must be marked active.
7. The primary drive must NOT be accessible.
8. The shadow drive's MBR must contain valid boot code.
9. The controller of the shadow should have the BIOS enabled/disabled in the same fashion as the primary's controller.
To check the make/model/firmware, use Regedt32 to navigate to:
where (n) is based on the device number. Double-click Identifier in the right hand pane. The last character string is the firmware revision.
NOTE: If you are greeted with a STOP 0x0000001E, you need to pull the connector from the primary drive.