Microsoft has removed the M drive by default in Exchange 2003 because of problems related to backup and antivirus software running against the M drive. However, you can still use the .BackOfficeStorage namespace to access the Exchange information through file APIs.
If you require the M drive (e.g., you have programs that use the M drive to map the Installable File System, you use Microsoft FrontPage Extensions and the Microsoft Web Storage System–WSS), you can enable the drive by performing the following steps (but do so at your own risk):
Log on to the Exchange server as an administrator.
Start a registry editor (e.g., regedit.exe).
Navigate to the HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesEXIFSParameters registry subkey.
From the Edit menu, select New, String Value.
Enter the name DriveLetter, then press Enter.
Double-click the new value, set it to M, then click OK.
Close the registry editor.
Restart the Exchange server (or restart the Information Store service) for the changes to take effect.