Long story short, I decided to try USB boot this evening.
New Midsummer Raspbian on SDCard, add line to program USB boot, all easy.
Copy OS (actually Ubuntu Mate in this case) to WD 1TB USB HDD -- not so easy. Had to mess with Partition Manager to delete the previous partition and put on a FAT32 partition; so the Windows would recognise the drive and I could write the image using Win32DiskImager. (Etcher still could not see it.)
Booted SDCard, all OK. OTP programmed OK. Raspbian can see the HDD, apparently properly written. (I'm a bit cautious because Imager baulked, but it seemed after everything was copied.)
BUT Mate will not boot. Not even a rainbow screen. Presumably the boot files are too old. Does anyone know if I can make it work by copying boot files from the SDCard to the HDD? Which ones? Any edits necessary?