Helpful suggestions all Tony, but the last one is the key.
When I try to export a Nikon D500 Raw + JPEG file from 2012 in Photos from either her MB Pro or my Mac mini using “export 1 photo” as a TIFF, 16 Bit, Full Size to a Documents sub folder, I get the “Export Complete with Errors” window showing the file name and error “the operation couldn’t be completed. (Cocoa error -1.)”
When I export the same file using “export unmodified Original for 1 Photo” with Export IPTC checked and sub folder format “none” I get both the JPEG and the RAW file successfully exported.
If I select the same file in Photos (tagged top left as “RAW + JPEG”) and press return I get a “Cannot Start Editing, Photos cannot load adjustments for this image” box. According to one thread on another site I should be able to select which file (RAW/JPEG) I want to edit when I hit enter.
Looks like only solution is to export combined file (RAW and JPEG) then use the separated files. Same apparently goes for old RAW only files.