I have an 2012 XFs portfolio with a very nice B&W sound system that can connect and play content from iPhone, USB, play CD/DVD's, and provides a really good DAB radio. Anyway, from day one I could never get the USB stick interface option to work, even if the same USB drive could be read by everything else in the whole world!!!!. All I would see is a list of the indexed files but from then on a very unhelpful help screen telling me the files were not accessible or something along those lines.
I could copy/burn the same music tracks to CD/DVD and everything worked fine. It was driving me nuts, i tried different manufacturers drives, various formatting options, using folders or not as well as a variety of drive capacities. Nothing seemed to work and then one day i was messing about on my PC converting to and from different music formats and downloaded a product called Switch plus to my iMAC. As an after thought i tried copying an existing MP3 album to a new USB drive via this Switch app but running the convert to MP3 as this wold take the original MP3 files and apply it's own MP3 conversion. Well knock me down with a feather, it worked?
What seems to happen is that the syncing or copying process used when downloading MP3 music appears to set a flag that the Jag's decoder doesn't like. I use Amazon music downloads so they're number one suspect but it could be iTunes or Sonos or even spotify doing something when these programmes search and catalogue the music found on my PC or network. So there you are folks, i know a number of people have reported problems, a simple solution is download Switch and copy all files to USB through its conversion process. The full, download only, product costs £18