Files are not recognized by a PC. What to do?

I (myself an IT'er) have a very strange problem…

I have various files I need on a hard drive (internal).

If I now want to search for these files with my computer, they are not available on the disk… Now I have taken the hard disk with me and connected it to the laptop with a hard disk case (USB).

Here I can see all files again. But when I connect it to my main computer again, the files are not displayed again… (exactly the same files as before).

No, they are not hidden files. And even if I did, I would see these on both devices. I even ran Recuva over the disk on the main computer, which couldn't find the files. (Which also makes sense because the files are not on free MFT memory).
Now the question of all questions is:

What can I do or what could be the problem?

Because even after restarting the PC, the files were not available. And when pulling out (hotplug) and plugging in the hard disk again, the files would have to be read in again, which would also mean that the files would have to be displayed again…

Maybe a permission problem? That you don't have permission to go to the directories on one computer?

Nope… I'm an admin

You may still need to take ownership before you can see the files.

Click on the properties, take ownership and give yourself the right permissions again.

An IT specialist is already writing where this data is and what happens when you display the properties of the folder. Go to the Security tab and look at the permissions.

And very simple, download Everything and Treesize Free and install both. If you start both of them with administrator rights, you will see all the data on your hard drive. Everything requires NTFS, Treesize Free also works with FAT32 and exFAT.

Or activate the deactivated administrator account and log in with it and access the data.

Does the problem only exist via Explorer or also via the command line or PowerShell?

I haven't actually tried that yet

I'll give it a try when I'm home again

I was the owner from the very beginning… And no program is authorized to explicitly rewrite this data

I'm already an admin, as I said…