Well, I formatted the USB drive in ext3 and copied a Mythbusters episode to it: it took about eight hours! This is the longest yet!
I have no idea why the Dreambox should be so slow when copying to a USB drive. I am quite convinced that copying the same episode from my Windows laptop to the USB drive will only take a matter of minutes (but I haven't tried).
It seems that I have lost this one, and I will need to get out the cables whenever I want to copy something.
Thanks, guys (and gals if there are any) for your comments and help.