Just received my new SD/USB Adapter - Sandisk SDDR 121 G35 (super compact little thing! Nice!) and it booted perfectly - I didn’t even re-burn the image on the uSD card, which was created using the Kingston adapter into my Windows PC - just popped it in and it booted straight up.
Incidentally after the fact, I also tried the micro SD card in an SD Adapter in the Kingston USB Reader and that booted fine too; so is only directly in the micro slot that the problem exhibits; which kinda suggests a connection issue - however there is no issue either writing to the micro or reading it in Windows, it is only the boot recognition that seems to be a problem.