Sansa Clip+ wont play my mp3 files if copied under MSC USB Mode

MP3 players Sansa Clip | Clip+
1

Hi I have upgraded the formware to to the latest version and my player stopped playing my mp3 files.

Here is my previous post: http://forums.sandisk.com/t5/Sansa-Clip-Sansa-Clip/Sansa-Clip-player-skips-tracks-with-scratchy-sound-after-latest/td-p/289612

However, what I found out - is that the player plays copied MP3 files just fine when the USB Mode is set to Auto or MTP.

If the USB Mode is set to MSC (Mass Storage), same MP3 files copied under this USB Mode cannot be played back.

Any idea why? I used to copy files using MSC mode before and play them just fine. 

Thanks a lot.