At this point, I would be tempted to format the player, reset to MSC mode and load it up again. Some of the confusion (and/or ‘missing’ files) could be that your computer can only see files loaded while it the same USB connectivity mode in which they were originally transferred in. In other words, files loaded while in MTP mode (or possibly Auto Detect which defaults to MTP mode) cannot be seen while in MSC mode, and vice-versa.
I’ve noticed too sometimes there are sometimes 2 Music folders, one inside the other. I simply move the files to the uppermost Music folder and delete the (sub) Music folder.
The Class 10 card could be causing some of the issues; they’ve been known to be problematic with Sansa Players, as they are designed for high speed and HD cameras, not mp3 players.
Another thing that could be causing some of the files to not show up )on the player) is the incorrect format of ID3 tag in the files. ID3 tags are what the player reads during the database refresh and then displays that information in your Artists, Albums, Genres, Songs, etc,. lists on the player’s screen, _ not _ the file names. If the player cannot read them, it simply does not display anything.