You can also buy an external 2.5" hard drive case for the laptop HDD, which will turn it into a portable HDD connected via USB.
Thats what I did with my old Creative Nomad mp3 player - pulled the HDD out and put it in a $16 USB external case, and turned it into a 40Gb external drive.
Eh, 40Gb was impressive back then
The biggest benefit of using a 2.5" (aka laptop) drive is that its fully powered by USB, and thus works just like a USB thumb drive or whatever. No external power supply needed.