I'm using a 20Gb external hard disc unit that takes power from the 3k via the USB cable.

The USB on the 3k supports most simple USB devices, so long as they don't use a custom driver.

Therefore it wasn't a shock to find it won't recognise my Pocket PC Dell Organiser as a USB hard drive, for example, which would have been rather cool as a backup device!

Most thumbdrives should be fine, even USB2 ones (although they will work as USB1 speed, which is still more than enough for midi file playback).

To be totally sure it is best to format (initialise) blank media using your 3k rather than (say) a PC.