My web server is losing its first hard drive - and now I need some help.

My setup is twin 18-gig drives.

My primary HDD has the traditional SYS partition on it (10 gigs) - and
that is mirrored to a partition on the second drive.

The remaining 8 gigs on the primary HDD holds the traditional CACHE
volume for proxy services.

I need to replace the primary HDD - and, I understand how to create a new
CACHE volueme on the second drive (that's not the issue).

My problem is how do I handle the mirrored drives - when do I break the
mirror, how do I have the computer see the second drive as the primary,
is it at this point that I remove the failed primary HDD, then how do I
set the mirror back up again...

Any help would be appreciated.