Hi sbv3000 - thanks for the tip. I have switched to using the image at the end of the "bios reset" bug thread, made sure there were no HDDs in the boot list (and moved the USB-HDD to the top of the HDD priority list in case it mattered), no sign of a UEFI setting, and reboots and now it works fine. Several reboots and shutdown/restart all working fine. The reboot is now much slower than before but that is not a big issue. I also tried the stick in different ports and they worked fine also. So yes it looks like I had the "BIOS reset" bug!
Many thanks