1. Dismount the databases which reside on the drive you want to change
2. Open computer management and change the drive letter to whatever you want
3. Open the Exchange Shell and use the Move-DatabasePath CMDLET with the -ConfigurationOnly switch
4. The command would be
Move-DatabasePath -Identity “Database Name” -EdbFilePath “X:\Exchange Databases\DB1.edb” –LogFolderPath “Y:\Exchange Logs\DB1” -ConfigurationOnly
5. You should now be able to mount the databases again because Exchange will have updated itself and look for the new drive letters
6. You may need to restart the MS Exchange Search Indexer service for the index files to start appearing on the new drives instead of the olds ones, or leave it to catch up!