The box was unchecked as I only let MC move files which are a part of the library.
Perhaps it was specific to a certain version, or when moving a certain number of files.
I've done a lot of reorganizing of my media recently in preparation for adding some more drives and have been left with hundreds of missing/stray sidecar files.
I did just test a single movie and it worked, so I don't know if that means it's working in the current version or if there's some specific thing which causes it to fail.
I suspect the latter, as I have not been able to determine a pattern for which files had their sidecars moved along with them, and which ones did not.
I recently did a rename/move of several hundred video files and all sidecars came along (as well as other minor moves), and I've not seen any "orphans" with the box unchecked. So it may have been a version issue, or there may be some other event which triggers the failure state. Some thoughts:
1) I know that if MC didn't create the sidecar/cover art, they sometimes get left behind.
2) File permissions can also potentially be an issue. I've run into this with JRiver for Linux (it may also be an issue with Macs since OSX is POSIX compliant) with the following fact pattern: if user A rips a disc and then deposits the files on the file-share on the file server, the file will belong to user A and have the permissions specified in the mask settings for the share. When a JRiver instance imports the file (assuming it has adequate permissions to do so), it will dutifully write the sidecar and cover art files, but it will do so as User B (the user that is running JRiver), and the file will have the permissions specified in the mask settings for the share. If User A and User B are the same user, there's no issue, but if they're different users, this results in the peculiar outcome that the media file and the "support" files have different permissions with all the consequences that that brings. That means that user A, for example, may be able to move the media file, but not the support files if he's running JRiver. It's easy to fix once you figure it out, but it's confusing as all get out until you do. It's definitely an issue in Linux, and from my reading, it may be an issue for Macs too (I haven't actually used a Mac since 1995, so I have no way of knowing personally)
3)Maximum file name/path length on windows can also be an issue (that's bit me in the past on a few files).
There could be other contributing factors?