Do you see the external USB drive in Menu>Settings>Storage? It should be listed beneath your SD card, if any is installed. Try going to /mnt/usb_storage and see if you can find your files there.
If that doesn't work or you don't see it, download "USB Mount All" from the market by Interphaze Solutions and it should detect it, then click on Mount and it should be available to view in File Manager HD then. That may be the issue (not mounted).