Thanks for the reply. Further investigation revealed that I had to disable the 'Secure Boot' feature in UEFI prior to booting from the USB flash drive..  The bootable USB flash drive contained the Memtest86+ stand-alone memory tester program (Memtest86+ binary is not signed by Microsoft). Obviously it would be much better to produce an error message plainly describing the actual problem (ie, "USB flash drive does not meet Secure Boot requirements" or some such message). Hopefully my experience will benefit others.
When I select a USB flash drive from the boot device list in UEFI (BIOS) instead a system diagnosis test is run which completes after ~8 minutes. A message is then displayed thus: "Success Hardware scan complete with no issues. Your machine will restart and download cSOS for recovery". Obviously this process is not booting from my flash drive. Can anyone tell me what I'm doing wrong and what is a "cSOS for recovery"?? Why is the boot process not running?  And why is a system diagnosis test run?  TIA
Unable to boot from USB flash drive