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