Sometimes only one of two external (ie, GoFlex, Backup Plus) drives mounts with a drive letter when connected to Windows.
This is caused if two drives have the same drive signature, which can occur when drives are setup in a mass production environment or when they are cloned.
The issue can be quickly fixed by forcing the drive on-line. To force a drive online in Windows:
- Press Windows key () + R.
- In the run box type diskmgmt.msc
- On the right, scroll down so you can see your external drives.
- You will notice that the 1st external drive is Online.
- You will notice that the other external drive is Offline.
- Right-click on Offline (in the grey area of the box) and choose Online from the menu.
The drive should show in Computer (Window Explorer) in a few seconds.