Yes this was a discussion on the Discord forum a few nights back. I’m not exactly sure where it is actually obtaining this 510mb “space” from considering that the /run/muos/storage path is directed to either SD1 or SD2 depending on what you have migrated to SD2.
I’ll be investigating it over the weekend to see what could be going on. For now the backup archive is a simple compressed archive, you can always split them up into smaller chunks. See the following FAQ for help regarding that.