Did you have the drive connected before or after you booted up? Mine prefers not to be plugged in until afterwards.
Have you checked your USB lead? Try the shortest one you have.
Have you updated the drivers for your hard drive?
You may need to assign a new drive letter - to do this
Click on start and type diskmgmt.msc on search bar. This would open Disk Management interface that is displaying all connected hard drives and partitions. Now select Drive from your disk management interface which doesn’t have any drive letter. Now right click & Select Change Drive Letter and Paths from the context menu. Click on the Add button in the next menu and pick a drive letter from the pull down menu. Click OK to assign selected drive letter and check your windows explorer or my computer to confirm missing drive of windows which was not showing you early.