Rpi USB drive won't load subfolder on mapped drive

Running Volumio (latest version) on RPi. I’ve been able to get the system working, but it’s intermittent. The main issue I have is loading the USB drive on my Win PC for ripping CD files to a subfolder on the USB drive. It will find the mapped drive but not load the subfolder.

Help appreciated!

Add: Can’t see folder using SSH - getting an error:

root@volumio:/mnt/USB# ls
ls: reading directory .: Input/output error


Reformatted drive with Mac to MDOS Fat (presumably FAT32). Created a folder, which is now viewable through SSH, but not in the Volumio GUI or when mapped/shared.