I have the following externals.
1x320 for taking with the laptop, nothing unique is saved to this drive, ever, unless it's something I copied to it from someone else, and that only stays until it gets home.
6x Freecom 500GB externals (containing Samsung Spinpoint F1s as it happens.
These were then backed up to........
6x bare Samsung F1s,accessed via one of these........

This obviously left me 6x500GB freespace on the new Sammys. The plan is that once these are full to bursting, 2TB drives will be affordable, and the cycle will repeat.
What are they full of?
The 1TB Sammys look something like this.....
Disc 1. Documentaries, about 85% full. with about 200GB devoted to all things railway.
Disc 2. Racing, almost full.
Disc 3. Backups from my windows desktop, linux desktop and laptop, along with backups from my brother's desktop and the fileserver, as well as installers and so on for essential software. about 70% full.
Disc 4,5 and 6 are about 75% full of lots of tv shows, a very few movies, my music, and a heap of books and audio books. Lots of full series, like all of Dr Who from "an unearthly child" to "planet of the dead", including audiobooks and comics etc (though these are not complete). All of Babylon5, all of Trek from Kirk to (the totally unwatched) Archer. Thunderbirds, Penn and Teller's Bullshit, CSI, etc etc.