Thanks. This helps a lot.

I removed the Windows features for old SMB, rebooted, tried mapping with IP and it worked. Thanks!

@johnnytwolegs It entirely depends on your network setup.

If you NAS is connected to the internet, then I guess it's a matter of configuring the firewall to allow access to the CIFS ports and making sure that file sharing is enabled for the internet interface. If you have a router in-between, you might need to forward the CIFS ports from the network interface to your NAS box.

Again, a configuration I strongly suggest against, since it exposes your NAS to the internet.

thanks, it took a while but now is done :)

@scully1888 why not put "everything else" on the SD card?

Changed fstab to:

proc /proc proc defaults 0 0
/dev/mmcblk0p1 /boot vfat defaults 0 2
/dev/mmcblk0p2 / ext4 defaults,noatime 0 1
UUID=1918-2A41 /media/usb0 vfat auto,nofail,noatime,users,rw,uid=pi,gid=pi 0 0

/media/usb0/BIOS /home/pi/RetroPie/BIOS none auto,nofail,noatime,users,rw,uid=pi,gid=pi,bind 0 0

/media/usb0/roms /home/pi/RetroPie/roms none auto,nofail,noatime,users,rw,uid=pi,gid=pi,bind 0 0

more streamlined and I think 'bind' in the first of the three additional lines was the cause.

OK it seems formatting to FAT32 fixed the issue. Thx for your replies and this great project, this topic can be closed.