I recently did this for my new powerbook. I bought a 25quid USB2 case and a cheap second hand 3.2inch drive from a colleague. LaCie do some software which backs up MacOSX. (Case was from Morgan's on New Oxford Street)
my cheap case came with a USB cable and a power cable. I think that you probably wont find a good one without external power - you will be drawing a lot through the USB cable and that may or may not work.
I just keep an image of my laptop hard disk so it just has to be at least the same size. Of course if you are doing something like rsync and using the backup software to keep a large number of incremental backups then you might want more disk space than your laptop.
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my cheap case came with a USB cable and a power cable. I think that you probably wont find a good one without external power - you will be drawing a lot through the USB cable and that may or may not work.
I just keep an image of my laptop hard disk so it just has to be at least the same size. Of course if you are doing something like rsync and using the backup software to keep a large number of incremental backups then you might want more disk space than your laptop.