All movies newly added to my external hard drive won't play on my Xbox one X through the...

https://support.xbox.com/en-US/xbox-one/apps/mkv-support

What type of media files are you trying to play?

Als be aware, that the UWP app version (i.e. the version that runs on Xbox One and Windows 10 from the App store, not the Win32/Mac version you can download from the VLC website) of the VLC Media Player does not support all media formats that
the Mac/Win32 app supports.

I have the same problem. The video apps don't update their respective libraries properly or in some cases at all it seems. What I do is explore the drive using a file manager app (I use File Explorer) which allows me to see all the files in each directory
including the root directory. Then I just select the file thereby opening it which then prompts you to select the app you want to play the video from a list that only contains the apps that are able to play that type of video file. The app takes a few seconds
to load on my OneX, probably longer on the other consoles, and video starts playing. Oddly even after playing it, it still won't show in that player's library.

The media files are either MPEG-4 or MKV. I can confidently say that the issue is not due to file compatibility as I have many other files in those formats that play just fine.

This is the exact issue: I have an external HDD connected to my Xbox with all my media (movies mainly) files that I have saved throughout the years. When you initially connect an external HDD to the Xbox it asks if this is for storing Games/Screen Captures
or if this is for Media playback. In this case media playback is selected and then all the files currently on the HDD will show up and play without issue.

Now here is the issue. After the initial setup and selecting this HDD as media only, any additional files I add to my HDD the Microsoft Media Player as well as VLC will recognize the file/folder depending on how I have it formatted, but:

1) If the file is not in a folder both programs will recognize the file but will not play it

or

2) If the file is in a folder, then when I open the folder the folder is empty.

This is a new issue and has never happened in the past.

One potential temporary solution I thought of, but I don't know how to do it or if it is possible: Can I have the Xbox forget the external HDD (without clearing the HDD obviously) so as to the next time I plug it in it will recognize the external HDD as
a new drive and I can select Games vs Media again? This way after media is selected all my new files will show up as well.

Update: I restarted my Xbox and reselecting the Games vs Media did not work. When trying to play an individual file (of the new files just added, because all my previous files in the same format work just fine)

Update #2: The external HDD I am using is a WD 8TB Easy store. As I stated above all my old files work but none of the new files do. As my issue was not formatting, I dug out my old HDD (a 2TB Seagate) and added my new files to that. All the new files played
normally. The question is.... why are new files not playing on my WD while they play normally on the Seagate. (BTW: Yes both HDD are formatted the same, exFAT). At least for me this appears to be an external HDD issue. I will do one more update after clearing
my WD HDD before adding everything back, but this will take around 1 day.

Update #3: After erasing all media and adding it back to my WD external HDD all files (including the new ones) play as normal.

The hard drive in the system is 500 gb. It can support a minimum 250 gb external hard drive. As far as I know you can store games that are compatible with Xbox One on an external hard drive but not music or movies. No one knows when the system will be
backwards compatible or if it will be at all. You can play blu rays but not digital copies of movies unless you go through a service like Netflix, Red Box etc that Xbox One supports.