Experienced users will check whether their external hard drive shows up in Disk Management when they find Windows doesn't detect it. To open the Disk Management tool, you can right-click This PC (or My Computer in Windows 7) and choose Manage in the context menu to enter Computer Management, and then select Disk Management on the left.

Once the drive letter is missing, Windows cannot recognize this drive but we can see it in Computer Management window. Now, we have to assign a drive letter to the HDD to make Windows recognize it.

Right-click the external hard drive, and choose Change Drive Letter and Paths…

Click Add, and then pick a letter for the drive and click OK.