Try this app,, it may help.... USBDeview
Find the USB Device that the drive is using, and right click uninstall it.
This is a great app to work out issues such as this.
But, don't go hog wild with it.
When you uninstall it (while the drive is unplugged from the system) when you plug the drive back in, it will redetect and reinstall the USB Adapter for the drive.
You also need to make sure that the drive letter you are using is not already in use.
This happens when you have a bunch of USB drives. They can overlap sometimes and cause this issue.
USBDeview can resolve it. Unless there is an actual hardware issue somewhere.
For the record,, changing the Drive Letter itself would not cause this issue unless you were able to assign a drive letter that was already in use, or something else took that drive letter.