External Hard Drive not accessible under XP OK in Vista

First step, Start, Programs, Accessories, System Tools, System Restore. Create a restore point and name it something like "Before USB Fix". This is to bail you out if something goes wrong during the following process and makes things worse.

Create a file with NOTEPAD containing the following lines and save it as FIX.REG

-------------------------- Use text after this line --------------------------------
REGEDIT4
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment]"DEVMGR_SHOW_NONPRESENT_DEVICES"="1"

-------------------------- Use text before this line --------------------------------

Double click on FIX.REG and say yes to the Merge Into Registry question.

Unplug ALL USB devices.
Open Device Manager.
View, Show Hidden Devices.
Uninstall all devices under USB Controllers.
Uninstall all devices under Disk Drives that you know are not present.
Uninstall all devices under Storage Volumes. Say no to any reboot prompts until you are finished. Also, if a Storage Volume doesn't uninstall, ignore it and move to the next one.
If you have a yellow ? with unknown devices, uninstall all of the entries there as well.

When this is done, reboot TWICE.

Reconnect the USB devices and see if they're recognized properly.

NOTE: If you have a USB keyboard and/or mouse, you'll have to modify the instructions and leave enough parts for those to function. I don't have one yet, so I haven't had time to modify the instructions. __________________
Remember: Data you don't have at least two copies of is data you don't care about.