Error -43 "...one or more items can't be found"

I was moving files from my GDrive external SSD drive to a WD external drive and it was going fine. Then, I dragged some folders and suddenly it said (no matter what files I selected and dragged) "The operation can’t be completed because one or more required items can’t be found.

(Error code -43)" Any ideas on the fix? Again I was dragging folders across (mainly videos and stills) and then it wouldn't do any more. I have a 2TB Western Digital external where I offload for storage files from the faster GDrive.