Thanks ManiacJoe. I have attached a screenshot showing Lightroom as well as the specs for my computer.
iMac version 10.15.7
Version 10 of Lightroom.
I'm using a card reader inserted into back of computer - usually works well and much quicker than connecting the camera.
The images are importing to a folder on an external harddrive (my usual process) but I'm still unable to see them in Lightroom. Even if I try another way and right click on the images, go 'Open with' ...Lightroom, then try to Add them without moving them. - So not importing from local folder/external hd to Lightroom either.
I've just done another shoot this morning on a different card and am trying to import the files but the same thing is happening; after 10 minutes it's showing only one image in the Lightroom Library and yet is merrilyy adding them all from the memory card to the folder.