Found one solution to Photos behaving badly after upgrade to Sequoia 15.1.1

Current system - Macbook Air M1 2020 (now on Sequoia 15.1.1)

Music & Photos libraries on separate SSD drives

Issue:

Upgraded to Sonoma 14.17.1 from Ventura and several applications stopped working properly, ie. Pages, Keynotes, Numbers, etc but worst of all Photos stopped importing pictures from iPhone or Macbook. Nothing that I tried worked.

Upgraded to Sequoia 15.1.1 to no avail. To be fair, all the other applications started working properly but the issue with Photos remained

I even went for help to the Genius Bar at my local Apple Store, they just suggested to 'freeze' my current Library and start a new one. That is the last thing I wanted to do

Solution:

Searching on the web for recent posts about this problem, I found several suggestions that the issue was due to the fact that Sequoia does not 'like' external drives that are not formatted as APFS

So I decided to reformat the SSD. First made several copies of my Photos Library (45000+ photos), terrified that I could loose my pictures then erase and reformatted the SSD, put the Photos Library back and it worked! Photos is now behaving properly, importing photos from different places without problems, keeping properties, etc

Hope this helps to resolve similar issues if you encounter them