The idea I had was if this is possible, I could attach one of the drives
externally via eSATA so it can be taken off site and in the event of a
server failure this off site backup could be plugged in and the server
could be back up and running in minutes rather than a good few hours of
re-installation (I was thinking of having two external drives so they
can be alternatively swapped every day).
You'll need a script to re-add the disk into the array to start the
sync, and yank it from the array before being unplugged, which is of
course easily do-able with "mdadm".
Of course as soon as you add the disk back into the array you're going
to cause a boatload of IO as the array resyncs from the internal disks
back out to the external one, and that's going to happen _every_ time
you plug the disk(s) in.
Are you thinking of this as the only backup? Recovery of accidentally
deleted files might be a bit fiddly if so.