I catalog my music using an application named Music Collector and it scans an inserted (or desktop mounted) disc to enter the pertinent information into a database. I have a number of older .sd2f files that I need to catalog. I can also enter it by title, but it is so much more efficient if it can be mounted. So it's more that I am trying to cut some corners!
Since you have been able to mount an image to the desktop in 10.12.6, then I may attempt to clean install Sierra to see where the problem lies. I have used XLD but not Picard, so I will try that as well. Thank you very much for the information!