Dragging jpg from Photos to external storage error.

OK so just got back from holiday and have around 1200 pictures over 2 iPhones that I have transferred to Photos on my Mac - all good.

Now when I 'drag' these 1200 pictures to my external storage it copies over around 120 pictures and then just stops.

Thinking it was just Photos not liking transferring to external storage, I created a folder on my desktop and tried the same thing again... but same thing happened. Its as if my Mac forgets what it is trying to do. I get no error message... it just stops.

I have searched online and have tried the 'repair your photo library' option... that didn't work.

I tried the export option from Photos, but this was taking absolutely ages (5 minutes to transfer 15 pictures????)

I used to do this countless times and never had a an issue on previous OS', so wonder if it is just Mojave?

Anyone had same issue that can assist as its doing my head in.