These are the types of USB-based equipment that I have and which doesn't seem to work at all.
USB CD-ROM
Tried a LITEON CD/DVD-R/W model.
USB HDDs 3.5"
Tried 2 Freecom Classic, both a 160GB and a 250GB,
and also a 'Sweex enclosure' with an older IBM HDD
I have not tried 2.5" drives, but don't really expect a difference.
USB Floppy drive...
Seems to kill most USB activity altogether...
Creative Zen 4GB.
Will take a charge, slowly, but so far I haven't had any other reaction...