Don't try to move your Photos library in macOS Big Sur. It will bring you only pain.
I have an M1 Mac mini with 1TB of storage. It has 430GB free space, and I currently have my Photos library (285GB) on an external drive.
I want to keep it on the internal SSD because, for whatever reason, the photoslibraryd daemon decides to thrash that external drive every time the screen is locked. It's like it decides to only do this when the computer isn't in use. Fair enough, but in the past three days it's written 185GB of data! WTF?!
Last night I tried to copy the file to the Pictures folder on my user account. It got to 244/285GB then errored saying there wasn't enough free space. LIES!
So, I tried again. It got to 241/285GB and said there's already a file called "Photos Library.photoslibrary" there, and promptly stopped. But, it didn't release the 241GB it had allegedly used up, but not actually used up. Nothing I did would change the free space, so I had to reboot...
Tried once more, same error: there's already a file called "Photos Library.photoslibrary" there.
Copied it to a folder within the Pictures folder, and it worked fine. Then I just moved it to the Pictures folder itself. THIS SHOULD NOT BE THIS DIFFICULT, APPLE!
Now, I've been into the Photos app and checked that all my photos are there - good. However, any empty albums that were in the original library no longer exist. Why is this even a thing? Why does moving a Photos library delete your empty albums?! Just another bug from Apple?
Into Photos preferences to mark this as the System Photos Library. Tick. Done.
No. The library file is now taking up 529.71GB on the internal SSD while the Photos app uploads 72,000 photos to my iCloud account, despite them ALREADY BEING THERE!
For *censored*'s sake, Apple. This process is made way too stupid by you and your developers.
1. Don't delete things from my library without my permission.
2. Don't upload 72,000 photos to an iCloud account where they already exist.
It's not *censored* difficult.