SQL - Emptying Secondary Data File
the 'SECONDARY' filegroup is full

Read Database Files and Filegroups.
To move all your data off of a File to other Files in the same Filegroup, use DBCC SHRINKFILE (N'SomeFile', EMPTYFILE);
To move your data off of a Filegroup to some other filegroup rebuild it targeting the new Filegroup, using ALTER TABLE REBUILD for a heap, or by droping the clustered index and recreating it on the new filegroup.