That certainly is strange, given that it had been working normally for 4 years. Don’t know if it will help, but it can’t do any harm is to update (or reload if current) firmware. Do it manually. Here’s a link to the most recent firmware version for your player:
This won’t affect the content, but will clear out any glitches that may be occuring in it’s operation. Remember to remove the SD card before updating the firmware. Also note that the USB mode will revert back to the default Auto Detect mode after updating. Leaving it this mode has never been recommended; it’s better to use either MTP or MSC mode manually. You will want to use the mode that the player was in when you orginally transferred your files onto it in order for your computer to “see” them.
