Which hardware for saving backups?

[Mitja Podreka <mitja.podreka@gmail.com>]

hello

I have a web server with about 270 GB of space on the three disks in 
RAID5. It's only function is to host a Moodle e-learning system.
I would like to make an external backup of user/course files. Not just 
for an extra level of redundancy, but also to protect the users from 
their own errors.
I've read a lot about backup software and already decided about which 
one to use. I would like you to ask about advice about hardware.
Is external USB disc suitable for this? Should I put an extra disc to my 
workstation? Should I make an "dedicated" backup server out of an old 
computer and new disc(s)? Should I try to buy some "ready-made" solution?
Going through a lot of hardship I could probably convince the faculty, 
to invest into some storage solution, but since the server is one of the 
only two Linuxes (Debians) on the faculty (the second one being my 
workstation) I would like to offer them an interesting/cheap(er) 
"OpenSource" solution.

thanks
Mitja