Error Importing Images into Photos

Where exactly is the folder located, you are trying to Import from? Is the folder inside a photo library package or on a backup volume? The filenames are looking like the names of working copies from a Photos Library. You cannot import directly from a subfolder of a Photos Library or from a folder on drive that has been used for Time Machine backups. You may want to try, if you can import the missing files, if you move them to your internal drive into your Home folder.

Ok., and the folder with the images is a simple folder, no longer inside a library or in a shared folder? I am asking, because I am getting the same error as shown in your screenshot, when I try to import from an originals folder or Masters folder inside a Photos Library package or from a folder in Users > Shared or from folder that is on a volume used for backups, for example by Time Machine.

Ok - at this point I'd try restarting in safe mode and retrying.

Then restarting normally and trying again.

If that doesn't help, I'd try a library repair

Repair your library in Photos on Mac – Apple Support (UK)

Hi

What format is the external drive? In finder, select the drive, and hit CMD-i

Hi

Where is your new library located? If on an external drive, what is the format?

What happens if you try the import in smaller batches of photos?

Where are the images you are importing?

What happens if you try file>import, rather than drag/drop? Again with smaller batches of images?

Are you able to check the ignore ownership checkbox (bottom of my screenshot)? If so, do so, and try again.

Hi

The external volume your library is on : is it (or has it ever been) used for time machine?

Also do you always have it connected to your mac before you log on, and continuously while you are logged on?