If mr_mischief is correct, I would say the simple but effective system("cp $from $to &") might be possible. BUT:

A single usb port on the pc has a transfer rate of about 40 Mbyte/s (source wikipedia). The slowest hard drives in hard drive tests in a PC magazine I could find had minimal 29 Mbyte/s write transfer rate, including notebook drives. Only Solid State disks got lower, the slowest was down to 17 Mbyte/s.

Now these were recent tests, older hard drives would be somewhat slower. But if you didn't especially look for slow drives yours probably will be faster than this minimum

So without buying an internal PCI USB card with a few ports the speedup you get is minimal to non-existant.