[CentOS] Mirroring Hard Drive

[Matt]

I have a 500GB Sata drive about 15% used I would like to make an exact
copy of too another Sata 500GB drive as a spare.  That way if
something happens to the one in service I can plug in the spare
quickly and restore one of the weekly backups without reinstalling the
entire OS and all the little tweaks of setup on this mail/web server.

How do I do this?  That is make an exact bootable copy of a linux
drive.  Its running Centos 4.6 if that matters.

Matt