[expert] Quick how do I mirror a drive?

[James]

We've got a Poweredge 2400 and the RAID was setup with 4GB boot and the
other 104GB were divided in half and mirrored.  We've outgrown the 50
user NT license, so we put mandrake on it.  The only problem is that
linux is just treating it as two large partitions.  How do I set up a
mirror.  We are doing nightly back-ups, but if something major happens,
it's much easier to switch drives.  Also, in NT there was a way of
setting the updating of the mirror.  Is this possible in Linux as well?