I have been able to determine that the failure is due to a lock on the database. The model I am using has two Table Select functions from an Oracle database into the same file geodatabase. The first one works perfectly (ie does not detect a lock on the database). The first Table Select is followed by a Make Table Query that joins data from the Table Select to an existing featureclass in the geodatabase and then copies the results to a final table using the Copy Rows tool. At this point, the second Table Select runs, but fails due to the database lock. I can only assume one of the earlier processes is putting the lock on the database. Does this sound likely - and if so, is there anyway I can release this lock from within the model?