Im surprised to not see anything about this on the forum, at least nothing i could find.
I have an AVIC-X9310BT, and using an SD card for music is a terrible experience for me.
IF i have my music divided into sub folders, it will only show some of the folders.
In addition, not all files within folders are found.
IF i have too many subfolders, it seems to endlessly load.
ie a folder structure like music/band/album/mp3files
IF i put all MP3 files into the root of the SD card, the unit will actually see and play the files, but again not all of them. It will only show between 200