Tente dar uma olhada em aqui .
A primeira sugestão é o que funcionou para a pessoa que pede uma solução;
Method 1
Check if both the external drives have drive letter assigned using Disk management.
a) Open Computer Management by clicking the Start button, clicking Control Panel, clicking System and Security, clicking Administrative Tools, and then double-clicking Computer Management. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
If both the drives does not have an drive letter then try to assign letters and check if it works.
To change, add, or remove a drive letter
a) Open Computer Management by clicking the Start button, clicking Control Panel, clicking System and Security, clicking Administrative Tools, and then double-clicking Computer Management. If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
b) In the left pane, click Disk Management.
c) Right-click the drive that you want to change, and then click Change Drive Letter and Paths.
d) Do one of the following:
i. To assign a drive letter if one has not already been assigned, click Add, click the letter that you want to use, and then click OK.
ii. To change a drive letter, click Change, click Assign the following drive letter, click the letter that you want to use, and then click OK.
iii. To remove a drive letter, click Remove and then click Yes to confirm that you want to remove it.