This might be of interest to other S3D users.
Whilst I know the shortcomings of printing via USB, that worked very reliably for me for several months, so I never got around to using an SD card.
Then about 3 weeks ago the USB became really troublesome and I couldn't print anything - so I started using SD cards.
However I'm now back with reliable (?) USB. It seems the reason probably was that about 3 weeks ago I did some serious rewiring of my 2X to replace a broken thermocouple - and the wiring harness was pretty untidy behind the bot. Wires going every which way.
A couple of days ago after replacing my broken mightyboard I tidied that wiring up, I still have wires dangling round the back, but they are now properly rolled up and zip-tied.
What I think may have been the problem with the USB failures was the fact that the untidy wiring was acting as a aerial and providing spurious RFI signals to the mightyboard.
Therefore anyone who might be suffering failures when using USB might want to review the wiring going down behind the bot - and invest in some wrap-on ferrite cores to go around the wires - those cores will soak up any RFI present on the wires and should given a much cleaner signal to the mightyboard.