You could have just copied the thumbnail database directories across manually, when MC wasn't running of course, which would be easy if you were copying them from one SD card to another.
They are stored in "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 23\Thumbnails" in Windows and have names like "{4F3F17FF-27D8-4413-9D09-A77EDF831ABC}", one for each MC Library. I'm sure you could find the Linux version on the SD card.
My only concern with that would be that the indexing of thumbnails was still relevant to the Library you are using. So backup the originals on your new SD card first (if any) and if the wrong thumbnails show up for Albums you will need to restore the backups, or just delete the directories and rebuild thumbnails anyway.