Got up outa bed to hit the can, and had to check back on the forums, lol. No, not on this player, but on the M50, I think. Maderin gave me the solution to the problem, I think, actually: folders with "hidden" attributes that also have been made "system" files (e.g. their "hidden" attributes box in Properties is grayed out/inaccessible) are not seen by the player's OS. This may be caused by a virus, although not on my system, AFAIK. But many of the files I have have been downloaded, and their attributes passed on, obviously; others have come from friends, not all of whom are PC savvy, let alone security savvy. So.
The solution is just to create new folders (with normal attributes) and to paste the files/songs into the new folders. So, problem solved! Mad spent quite some time today to work this through, and help me out, and I'm really grateful. Thanks for your interest, too!