I am posting this as a question in case others have this issue I had and maybe someone with a better understanding of it can even help me understand it better.
While migrating to Musicbee and organizing, retagging, and cleaning up my library, I would occasionally use the Backup function just in case I screwed up and lost some work...
I am coming from WMP and the problem was that the FIRST backup (to a new location... so all files copied fresh) would work fine. Then, after doing some more editing, if I tried another backup, it would be halted because MB was denied access to one of the Folder.JPG files (I think it was always one of the albums I had changed the artwork for). BUT, when I went to the location, the Folder.JPG file was hidden EVEN with view hidden files enabled. I came to the conclusion (but not really certain) that those files were also tagged as system files so could not be viewed or have their properties (such as read-only) modified. When I looked at the root music folder, it would have the "dash" in the read only box (meaning SOME files in there are read only but not all (which would be a check mark). Unchecking the read only for the whole folder never seemed to work. Again, I think because the folder files are attributed as hidden system files....
The solution I came up with was to update ALL the Album art with new pictures (the WMP ones were crappy resolution anyway) and have MB organize the library to a new location so that it could write it's own Folder.JPG files. I had a few more times where I missed updating the artwork (or maybe if there wasn't already a Folder.JPG file created by MB then WMP would write one with the read only properties?). Anyway, I stopped opening WMP and got all my art updated in MB with normal (not hidden, system) Folder.JPG files and haven't had the permission problem during backup since. BUT, if I do get it again, my solution will be to delete the offending album folder IN THE BACKUP location. This will delete the associated Folder.JPG file and the backup will rewrite all the deleted files anyway, including the new folder.jpg file.
This had me really frustrated for about half a day and I hope this information helps someone. If anyone really understands what was happening and want's to explain or provide a less "hacked" solution I am grateful.
I am using Windows 11, latest update and latest MB version.