iOS 26 fixes this with an option to only show photos not in an album.

Additionally, if you are backing up photos to a hard drive you will be much better off using an app like PhotoSync. It has a one time payment and transfers direct to whatever you want with far less errors and more control than Apple’s terrible buggy disaster that is transferring photos by wire.

It’s actually already available in iOS 18! On the main Photos page, click the little filtering icon and select something like “Not in any album”