|USB devices may not work correctly when they're connected to certain USB ports. Instructions for how to re-load your USB stack in 2000 or XP, if you need to - similar applies to Vista and Windows 7.|
In addition, all external drives connected by USB require that the port (or ports) they connect to can actually supply 500ma of current.
USB external hard drive requirements.
See Response 9:
The flash drive (or external drive, or memory card reader) must be plugged into a port it will actually work in.
How to tell if the flash drive (or external drive, or memory card reader) is properly recognized as a hardware device, even if you can't access it's data.
Programs you can try to recover your data, if that's okay.
The most frequent reason people can no longer access the data on a USB flash drive, USB External drive, or a memory card on a computer
See Response 2: