Enable M Drive in Exchange Server

You need to add a string value called DriveLetter, with a value of M, into the following location:

HKLM\System\CurrentControlSet\Services\ExIFS\Parameters

and change the Value of EnableMDLReads to 1.

Obviously, the value M means that drive M: will be used. You can change this to be a different value if you desire.

If you do re-enable the ExIFS drive letter, be careful out there!