external drive

Clarifications

This could either be hardware or security differences, but it's only a guess.

Try rebooting the computer without any USB devices attached. Then plug in the USB drive, right click on My Computer, select manage, and then click ?Disk Management.? If the drive is not present in the Disk Management window or if you are not able to access it, then the drive may have failed. In this window you should see all of your connected physical drives, their format, if they are healthy, and the drive letter.
Change the drive letter of the drive. Right-click on the drive in the list, and from the resulting menu select "Change Drive Letters and Paths"
Click on Change so you can change the drive letter. Click Change.
Select a new drive letter from the drop down list, preferably the one that is not normally used for this drive. EG: U:
Click Yes on the confirmation screen

To disable power management on the USB hub:
1. Right-click My Computer, click Properties, click the Hardware tab, and then click Device Manager.
2. Double-click Universal Serial Bus controllers to expand it, right-click USB Root Hub, and then click Properties.
3. Click the Power Management tab.
4. Click to clear the Allow the computer to turn off this device to save power check box, and then click OK.