No one suggested this in response to previous pleas for solutions so I post it up her to assist others. It is far too easy to include an incorrect album in a merge. Deleting the merged album and reintroducing the files from an external source is one solution that works but here is another:
Select the merged albums -> Edit -> Fix Track Grouping
Select and highlight the disk to be removed - all its tracks should appear on the right. DOUBLE CHECK ON THIS You might have to re-highlight to sponsor the correct tracks to appear
Check each and all of the album tracks - they will be highlighted
Select “Remove Tracks” at the top right
Select agreement to “Create Album (X discs Y tracks)”
Two covers will then appear on the Albums list page. One will be the merged group (or a single album if originally only two) without the removed album, the other will be the removed album with its original cover.
I stumbled upon this after using the remove tracks feature to remove the first track on some albums where there is useless talk/introduction of the artist. This creates a second “album” with the removed track as the only one there so that “album” is then easily deleted and the library cleaned up via settings to permanently nuke it.
Nice work Roon 