If you set your computer up with a Raid 5 array for example you would need 3 HD's in the case. One part is mirrored so effectively you only see 2 HD's, the other is used just like a storage drive if you like. So you could use 2 X 500 Gig HD's for the mirrored drive and a 1 Tera for the storage drive or any other combination, the mirrired drives have to be the same size though.

Why I hear you ask!!   If the main drive goes down, all your OS, Files etc. are mirrord on the other drive so that drive can take over which gives you time to get the other one fixed. You get a message to say something's wrong with the drive so it can be sorted quickly.

The advantage of this is that you are unlikely to lose data due to a HD failure, I lost important client stuff a few years back due to a HD failure, I've had a Raid set up ever since, it won't happen again.
I still do backups to an XHD as well, just in case!

Injustice will be avenged.
Cofiwch Dryweryn.