I suspect you are conflating • booting from an external hard disk and • reading an external hard disk.
As long as you can physically plug in the drive, you should be able to read from it, I think.
Alternatively, you might mean that the photos on the external disk were managed by an older photo-management application. Offhand, I am very confident that the photo files will be there, as files, regardless. That is… do not use the Photos application on the computer; instead, use the Finder to look at the files directly.