you could also add mount commands to batocera-boot.conf
something along the lines of:
sharenetwork_cmd1=mkdir -p /mnt/exthdd1
sharenetwork_cmd2=mount /dev/sdb1 /mnt/exthdd1
sharenetwork_cmd3=mount --bind /mnt/exthdd1/roms/windows /userdata/roms/windows
copy the last command to add a mount to another folder.
for example, to mount ps4 roms just add:
sharenetwork_cmd4=mount --bind /mnt/exthdd1/roms/ps4 /userdata/roms/ps4