I think it relates to SQL Server not allowing nested inserts in a Insert into .. Exec construct. That is why instead of using the "insert into master..space1 exec sp_spaceused " construct yank the code from the proc and use it directly, ie, make a new procedure that has the code from sp_spaceused and inserts into the table directly. And then you can call the code for each object, DB etc.

Or, use the sysindexes and do it for the entire database at one shot.