I had chosen to install from a USB key, but what I had forgotten is that the motherboard in the PC is a Gigabyte motherboard, and it doesn't like booting from USB. This is a common problem with Gigabyte motherboards - just Google for Gigabyte and "boot from USB", and you'll see many variations of "can't boot from USB", "won't boot from USB", "unable to boot from USB", etc, etc. What happens is that the motherboard just completely ignores the USB key; effectively, it's like it isn't there.
Unfortunately, there isn't a solution to be found in any of those posts. Or at least I ran out of patience before I found one, although it's clear that the problem is to some extent USB drive dependent - Gigabyte motherboards seem to like some drives, others not. So rather than continue to go through endless posts that don't offer solutions, I started playing around. What I found was:
- With any other drive in the machine, even if the boot from the drive was disabled in BIOS, the Gigabyte motherboard would still ignore the USB key and boot from the other drive;
- With all other drives physically unplugged, boot would fail with a "Insert boot disk and press enter" message. The light on the USB key didn't even flicker at any point in the process, indicating that the USB key wasn't even being seen;
- However, that was where things got interesting. What I found was that if I then unplugged the USB key, and plugged it in again, then pressed enter, the USB key was recognized, and the machine booted.
Thinking about this, the USB key only being recognized if it was unplugged and then plugged in again suggests that somehow, if the USB key is inserted at the time the machine is powered up, the motherboard gets into a mode where it doesn't recognize the key and it gets ignored until a operating system driver starts up. But later in the boot cycle, the motherboard does seem to be able to recognize it. So what I did was simple:
- I started the machine with the USB key unplugged;
- Once the machine had got through the first parts of its boot sequence, I plugged the key in, then hit the F12 key to bring up the boot menu. And on the third try, there the USB key was, on the list of hard drives. And it booted fine!
Now I must admit, plugging the USB key in during the boot sequence is neither elegant or reliable - you'll probably have to try it a few times - but it worked for me.