If you have all software installed and the drives mirrored, by "breaking" the mirror the second drive is an identical, virgin copy of the first. Disconnect it and it is a bootable spare, ready to install if the primary fails. You MUST back up your data, obviously.
If I remember correctly it can be done, but requires making a bootable drive first - then using the xxcopy/clone command. I know I have successfully cloned an XP drive, but also remember it was not as simple as W98.
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