Apple have issued a major update to Mac OSX called 'Big Sur' which replaces Catalina.
SUMMARY: if you have data used by apps e.g. Adobe products or music software on the same drive that is used for your Time Machine back ups, be VERY careful.
DETAILS
I got caught by surprise by this and wasn't paying attention and thought it was an ordinary update. There is a major "feature" of this change which is causing me immense problems and I'm writing this as an advisory in case you encounter similar issues.
A major change of this update is how Time Machine works. In my case, I have a 4TB NAS drive designated as the target to write the back ups to. My Mac is only 500gb SSD on board, so it's more than enough. I also use the NAS drive to hold the bulk of my documents and all of my files for photos used by Light Room and Photoshop. The RAW files from my current camera are 50mb each, which is why they're on the NAS drive. The LR catalog is on the SSD, so you can quickly search the thumbnails, but it has to go to the NAS to get the full image file.
Apple have decided that the drive that is used by Time Machine can ONLY be used for Time Machine. Personally, I do not see how they can assume such a right as it is MY device.
This means that not only is the drive not visible in Finder, but you can't even log into it via the NAS drive's own app (Western Digital in my case). Worse, LR cannot see the files and all 30,000+ photos are showing as 'orphaned' and detached from their files. That means I am unable to access or process any of my photos.
Apple have suggested one solution, I may have worked out an alternative.
Apple solution: this will be done with the assistance of a support person on the phone tomorrow... Restore the Mac back to Catalina and then recover to last image copy/restore point.
Given that this is over wi-fi etc I am not confident that this will work, if it does then 30 or 40gb over wi-fi will take forever.
My solution: I have ordered a 4TB external HD. I will plug that into the USB3 port on the Mac and designate it as the Time Machine target. Once it has backed up, I think the NAS drive will become 'free' and accessible. This does not involve recovering back or a system restore.
I think my solution is less risky.
I will provide updates on progress.