If you need to determine how much whitespace is in your Exchange database you can look in the application event log for event id 1221. It will say something to this affect:
1. Dismount your database and run the following command
C:program filesexchsrvrbin> Eseutil /d c:progra~1exchsrvrmdbdatapriv1.edb
2. Create a new database within your storage group and move all of the users’ mailboxes to the new database. Creating a new database means the database is optimized an compacted. So when the users’ mailboxes are finished moving you will have a completely optimized database.
The downside of option one is you are completely down while your database defragments and you need 110% of your end database size to be free. Before running option 2 do not forget to enable circular logging. The reason being is when you transfer mailboxes from one server to another you generate logs equal to the data you are transferring. These log files can consume huge amounts of space during this process.