Another strange thing is that some of the files in the same folder are in sub-folders and they open normally. It's only the loose ones that weren't put into separate folders inside the main one that refuse to open.
That's mildly interesting.
You might try creating a new folder structure on the new machine where you are relocating the files with folders or just a single folder which ever suits your purposes best and then attempt to recopy the loose files into that container or those containers.
It sounds to me lke an inheritance problem where some files (those loose files) were impacted differently by your efforts to relocate them.
It's somewhat complicated and the fact that you used a third portable drive (not the host and not the destination) between your efforts might have further complicated the process.
It has to do with drive formats (ntfs, fat32, exFat, etc) and how inheritance is handled (maintained, changed or discarded) in each instance and in some other instances how the file was relocated (copied versus moved).
You might try Google ( explain file inheritance copy move ntfs fat32 ) to help understand some of the pitfalls.
OR
You might try just one of the problem files.
Disable inheritance, do not convert existing permissions, just remove them from that single file, and....
Then add just the "Everyone" group and grant that group full permissions to that single file and see if that has the desired results.