Photos with unavailable original files cannot be opened.

I bought a new Mac computer and tried restoring using TimeMachine from a previous Mac -- but have had trouble with only SOME of my photos -- I keep my photos on SSD external drive connected to the computer -- the SSD is the set location for photos and is the system's designated library. I notice that many of my photos are not available (although they ARE found and I can see them) when searching in Spotlight on the photo app. (Settings within Photo app says that the external drive in not searchable.)

When I click on the photo I am looking for, the error message I receive is "Photos with unavailable original files cannot be opened. The original photo “Sunset on Lake.jpg” is either offline or cannot be found. Click “Find Original” to reconnect."

When I physically go to the SSD external drive, I can see the photo there -- BUT, within the error of the original message, if I search for the original, the external drive is not even accessible - only an icon of it is there.

Any thoughts? Thanks!