So I've encountered a big issue on many of my phones and different firmwares.
When copying files from my computer on my phone with MSC mode, I've lost many files, others were corrupted, others were in LOST.DIR folder but not restorable, and finally, some folders were on the card but not accessible, anyway, a big issue.
Some of these copy issues disappear temporarily when rebooting the phone, when others have no solutions...
But good news : I've found a workaround.
If I unmount my SDCard from the phone before activating Mass Storage mode in the app, there is no problems at all, AMD I've tested that for months !
I've got that idea of unmounting SD Card because on my older phones, before MTP mode was predominant, I used MSC mode and it was unmounting the SD Card on the phone , which was never connected on both phone and PC.
So I suggest that you add a SAFE MSC MODE in the home of the app (or as an option, check box...), which would automatically unmount SDCard as soon as Mass Storage is activated, and which would REMOUNT Sd Card when MSC mode is deactivated in the app.
I really hope it's feasible, and thank you for that great app.