I have a 250GB portable 2.5" Seagate drive in mine.
Had to use a tool to format the drive in FAT32 (wont read it in NTFS).
65GB of music, no problems!!
Although the MIX (random) does not stay on between ignition starts and I think it says somewhere in the manual that it will only randomize the first 1000 tracks.
Anyway, sits nicely in the tray area where the USB port is so you can't see it if you don't know it's there.
I got it from Argos as it was cheapest around at the time.
cat num is 675/8648