I observe the following error when using mp3tag in a folder that is synced by Google Drive's synchronisation client "Backup & Sync":
Open a folder with media files
Open "extended tags" on all files
modify embedded image (e.g. add or remove image)
from here on, it's a bit tricky to reproduce
wait a bit, until google drive has synced about half of the files that were modified
modify images again, like described above
Most likely there will be an error on some files that says, mp3tag couldn't write-access the files. The image modification will only be applied on some files.
Unfortunately, it's a bit hard to reproduce. Often, you have to repeat the process described above several times. Wait longer or shorter between the modification. I'm not sure if it has an impact.
If you see the error on one particular file once, you will always see it on any further attempt to modify the file. Even when the Google Drive synchronisation is done, the error will still be there. Until you completely close Google Drive Backup&Sync.
To me, this sounds more like a bug in Backup&Sync, so I contacted them via Google One Support. But they keep on telling me it must be a bug in the third-party application, namely mp3tag. So I just wanted to describe the bug here, maybe someone else can reproduce it and maybe we can collect further information to prove wheather the problem is on Google Drive's side, or on mp3tag's.
If you need more information to understand the problem let me know. Then I will provide some screenshots.