Hi! I have my psx ROMS in a usb hdd. The frontend can see them and are playable, this isn’t the issue. The problem is that the system gives a timeout error when starting with the hdd conected, and before this, some emulators dont work properly. This is the error code:
udevd: timeout: killing ‘/usr/share/usbmount/usbmount add’ 
udevd: ‘/usr/share/usbmount/usbmount add’  terminated by signal 9 (killed) done
try putting your command into rc.local, because i had an issue where some things had not initialized on startup, when using an ftp mount, so putting the mount command into rc.local helps. if you want to, you can try putting a delay before that in rc.local too.