"Specify the Output File( name it gmapsupp.img)"

If I set up an output directory to join 2 mapsets It fails on trying to create the gmapsupp.img "Create output file error, file c:/gar/output/gmapsupp.img."

Do I somehow create a blank gmapsupp.img and set that as my output file to join these into?

Or do i join one mapset(topo) topogmapsupp.img into an existing gmapsupp.img that would contain my other mapset (Bluechart from my SD card)