Photos not loading from external Photos library on Mac Mini

Device: 2018 Mac Mini, Software Version: Sequoia 15.7.2

I am trying to access a large Photos library from an external hard drive. There are random photos not loading across the library (showing as a grey rectangle). iCloud not used as backup. This is my husband's computer and he is now unable to use. I just want to access all of the photos so I can download and save in a different format. Thank you.