SQLServer probe is returning incorrect space calculations for the following metrics:
The space calculations may be inaccurate, empty or display negative values.
SQLServer 2008 - 2014.
This is due to the sp_spaceused system stored procedure returning incorrect data. This is most common when the database has had Data Definition Language (DDL) modifications, such as CREATE, ALTER, or DROP statements.
Run the following command to recreate the statistics for the database. Note this can take some time to run on large tables or databases. DBCC UPDATEUSAGE (DatabaseName)