Back up problems - Lightroom catalog

This is sometimes caused by catalog corruption. Create a new catalog and check if this catalog can be backed up normally. If it can, then in the new catalog, choose 'File - Import from Another Catalog'. Import your old catalog. Import all images but do not move them. This creates a copy of your old catalog. After the import is done, check again if you can still backup this catalog normally. If so, then start using this one as your catalog.

Please note: The only thing that won't come across is any publishing services you may have. You will have to recreate those. If you synced your catalog then you will have to enable sync for the new catalog. There are some reports that this could cause a problem. Here’s how to solve that: https://www.danhartfordphoto.com/blog/2025/1/lr026-fixing-sync-problems-caused-by-switching-the-sync-catalog

The message also says there may not be enough space on both drives involved. Have you checked that?