I am no expert but found this thread via Google having experienced similar problems with both a Western My Book and a Seagate external hard drive. After turning off the computer, disconnecting all USB devices, then rebooting and connecting each USB device in turn, the problem turned out to be my newly-acquired HP OfficeJet All-in-One. If that is on and connected when I turn on the external hard dive, the latter is not recognised by Explorer and just keeps whirring away (and, the case of the My Book, flashing). Turn off the printer and wait for its USB connection to terminate and the hard drive works perfectly without more ado. I can turn the printer back on again (or just reconnect it) without causing any problems with either hard drive but cannot turn the hard drive off and then back on again without having to go through the same procedure. I do not think there is a drive letter clash as I have given each drive (including the multi-card drive built into the printer) its own drive letter via Disk Manager and XP remembers them. Perhaps someone more expert than me can interpret this experience into a generally applicable "fix"