MS have written a KB article on how to do it. Q239747 How to Change the IFS Drive M to Another Drive Letter, shows how to do it.
Basically look in the registry for HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\EXIFS\Parameters Look for a value called DriveLetter. If it’s not there then add it as a REG_SZ data type. The value is the actual drive letter you want the Exchange data to appear on.