One important tool in managing disk space is to make sure the system accurately reports space used by tables. DBCC UPDATEUSAGE will update the values returned by sp_spaceused.
The output of sp_spaceused “displays the number of rows, disk space reserved, and disk space used by a table, indexed view, or Service Broker queue in the current database, or displays the disk space reserved and used by the whole database.” One thing to note when reviewing the output is you may see the the reserved size is disportionately larger than the data and index space used. The reserved space should always be greater, but in one case I did see > 20 GB of difference. The space reported by system can get out of whack (technical term) from repeated use of TRUNCATE statements and frequent DDL statements.
Excellent article on DBCC UPDATEUSAGE by Armando Prato.
Google search for DBCC UPDATEUSAGE.