If your drive works off of only usb power, make sure you are running a fairly new computer with usb 2.0, as old ones only put out a low current which doesn't allow enough power for the external drive. Also check to see if your drive is trying to grab a drive letter which is the same as another device's that you are running on your computer. (Are you running more than one external drive or even a network drive?) Check this by stopping all the other drives (unplug network cable for the network drives if you don't have full administrative rights) and start your drives. Edit the drive letter to something weird such as m or q in the disk management app. Then restart your other resident drives.
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If your drive works off of only usb power, make sure you are running a fairly new computer with usb 2.0, as old ones only put out a low current which doesn't allow enough power for the external drive. Also check to see if your drive is trying to grab a drive letter which is the same as another device's that you are running on your computer. (Are you running more than one external drive or even a network drive?) Check this by stopping all the other drives (unplug network cable for the network drives if you don't have full administrative rights) and start your drives. Edit the drive letter to something weird such as m or q in the disk management app. Then restart your other resident drives.