On my HP ProDesk Mini Tower, the boot menu lists UEFI and Legacy boot entries for all drives. But it lists all the UEFI entries first, then lists as many of the Legacy boot entries as will fit the (non-scrolling) menu. It truncates the combined list at 15 or so. So, when I tried to boot from the thumb drive that I had prepared, the UEFI boot entry appeared among the entries at the top but there was no Legacy boot option for that thumb drive listed at all. I had to remove one of the other USB drives, thereby shortening the list, in order to have the Legacy boot option appear.
As always, your mileage may vary.
As always, your mileage may vary.