Partial export of images from Photos

I'm trying to export large sections of my 31000 image Photos database to an external folder as jpeg files, without using subfolders i.e. just a straight list.

Before I upgraded to High Sierra on my 27"iMac, the standard Photos process worked fine - select the photos and use the Export command to export them to a specific folder. Post the High Sierra upgrade, I can only export 200-280 files into a dedicated folder on the Desktop. (the number varies each time I try it). These files in turn seem to be a random subset of the selected files. The progress indicator in Photos however still shows activity but there is no indication as to where the photos are going, if anywhere. Eventually the progress indicator completes and everything stops. One particularly strange thing is that I get two separate progress indicators, one after the other; the first one moves very fast and loads about 20 or so files into the folder; this then completes and the second progress indicator appears, continuing to load the folder until the 200 or so files are achieved, yet continuing to show progress after that at a speed appropriate to the number of files selected despite no more files being loaded.

The Photos database is on a separate USB 3 hard drive which has been faultless on Sierra. I have rebuilt the Photos database, run First Aid on all the drives involved, and reloaded High Sierra. I have deleted the Photos cache files. Any ideas as to how I can fix this annoying problem are very welcome.