Library on Synology NAS USB device

I'm running the latest serviio on my synology ds411+. Love how it works.
I have no problem playing videos with the library stored on the internal synology drives, but when I try to store video files on an external usb drive, it doesn't seem to find the files. The folder is shown, but it is empty.
Hopefully I gave enough information. Can anyone tell me if this should work or not?
Thanks,
James
I have no problem playing videos with the library stored on the internal synology drives, but when I try to store video files on an external usb drive, it doesn't seem to find the files. The folder is shown, but it is empty.
Hopefully I gave enough information. Can anyone tell me if this should work or not?
Thanks,
James