Some MKV files play from computer but not from MyBook Duo

Hello! I really don’t have knowledge on networking and systems, so I would really appreciate some help in laymans terms. I have a MyBook Duo external drive connected to a Mac Mini. I have watched hundreds of mkv files that are stored on this drive, via my mac: through either Plex or VLC player directly on my Mac. Just recently though a bunch of my mkv files (not all of them, just some, but a quite a bit) won’t play if I try to play them off of the MyBook Duo. So I have all the files stored on this external drive. If Itry to watch in VLC, it says “File reading failed VLC could not read the file (Device not configured).” And if I try to watch through Plex, it just shows the loading symbol and never loads.

Again some MKV files are playing just fine off of this external drive.

However if I move the files to my actual Mac mini- they play just fine through VLC. (and I haven’t tried connecting Plex to my local computer storage folders as I don’t really know how to set that kind of stuff up in Plex and don’t want to mess up the connections I have already that someone else helped me set up, whom I don’t have to help me anymore.)

Anyone have anything that could help me solve this? Thanks.