They're 2.5" SATA drives but you can't just buy any drive and slot it in. You can however get any Xbox 360 hard drive (available in capacities of 20GB, 60GB, 120GB, 250GB and 320GB) and if they're in the older size enclosure for the older original Xbox 360, you can take the drive out of the enclosure and slot it into the 4GB 360.
This.
You do however need a means of keeping the drive straight in the slot itself (the slot is designed for a caddy not a bare drive). I use some pieces of packing foam to keep the drive straight in my 4GB (had a 120GB Elite that died on me).
Yeah getting a caddy isnt hard, however, what is the 'special' requirement that means you can only use drives from other xboxs? Why can't you just use any old SATA 2.5" drive?
Firmware on the drive.