When attaching an externally powered hard drive to the bottom USB port, both "fdisk" and "mount" struggle for a long time, and once mounted, reading from it is extremely slow. Using the top USB port, however (which is what I happened to plug my USB hub into when I first got the Pi), it works flawlessly. I first assumed it was a hardware fault, but when I tested my other Pi using the same SD card, the behavior was exactly the same.
They are both rev 1. I'm running up-to-date Raspbian and firmware.
Any idea why there would be a difference?