I have my main Photos library on an external SSD, and it works fine except for one thing: from time to time, perhaps once a month, it suddenly does not „see“ the library any more. When I restart Photos, it says the library is used by another program and I should chose another library which is not helpful of course. My fix is to restart the iMac, and then it is ok again for some time until it occurs again. The funny thing is, that all the other data on the SSD is still accessible, so it is NOT a connection problem with the SSD. It is only Photos which is affected. Any ideas?