Alex, you're a saint. You've helped me before with similar issues and your assistance then was invaluable.
The first two paragraphs of your reply were similar to the way I do things usually:
With most of my ripped movies etc., I download the DVD cover art and do the MC/Explorer side-by-side tagging method like you suggest. In fact, it was your suggestion from a previous thread of mine.
With captured TV shows, I usually take a 'save screengrab as thumbnail', and more recently I've been saving the screengrab to a file as well, for backup. I like the 'save screengrab as thumbnail' as it's a one-step option. However, I cannot choose where the screengrabs end up, so I'd resolved to save to a file and then do a side-by-side cover art tagging.
The latter part of your reply (and edit) are going to be very useful to me next time I do some tagging. I've never used an mpl file before, so it'll be a learning experience. Thanks a lot!
Now that I've calmed down a bit, I think I know what's been happening. A couple of weeks after tagging video files with external cover art images, the way you outline above, I found that the image file field had been changed to a date! I had meticulously arranged all my video files and tagged the album field appropriately too, in case there was any chance that MC would automatically rename my cover art to the old [album]-[artist] nomenclature. I've been running MC13 on another computer and using backups of my HTPC library on it. The files themselves are in shared network folders. I'm thinking that maybe MC13 had been renaming my image files because of its settings and then saving tags in the shared folders' video files.
I'd like the MC13 preview computer to open libraries in read-only mode, without having to have the HTPC on all the time. Is that possible?