Go into Disk Management to see if it is shown there.
If it is, it may just need a drive-letter assigning to it, as Windows often fails to do this automatically with external hard drives.
In the bottom-right panel of Disk Management, right-click the ribbon which relates to that drive (it usually says "Raw" on it's ribbon).
Choose "Change Drive Letter & Paths".
Click "Add".
Choose letter.
Click 'OK'
Wait for Disk Management to update itself.