Well, it all depends on exactly how much you're expecting.
Read from XBOX to PC will be fast, as the drive heads only have to read what's on the XBOX drive, and then your PC handles writing it to its own drives.
Writing to the XBOX drive will be decidedly slower, since not only does the box have less ram to handle such activities, but the drive is also an ATA33, so the write speed (inherently slower than the read speed) is going be slower than your computer (probably ata100/133)
Now if you're getting less than a meg a second write speed, I'd check if you have a firewall blocking you from the xbox directly. (Having only the ftp open at the time will yield slower reads/writes than just trusting the xbox's ip completely.)
-Splitch