Probably no Drive Letter has been assigned to the hard drive.
Assuming you are running Windows OS, follow the following steps -
1. Click on Start button.
2. Type diskmgmt.msc. Hit Enter.
3. In Disk Management console, your device should show up.
4. Right click on the disk drive. Click 'Change Drive Letters and Path'.
5. Click on Add/Change. Assign an alphabet.
6. Hit OK.