Why does the flash drive boot the system GRUB2?

Question

linux

Hello. The point is. A laptop.
2 systems (workers) hang on hdd: Manjaro Arch and Win7.
In general, you need to boot from a USB LiveCD, the system image has already been written to the USB flash drive (it doesn’t matter, any, Mint, Ubuntu, it’s only important to get into the LiveCD).
When you try to boot from USB - nothing happens, in general, grub seems to take over the initiative.
There are flags on the flash drive. Recorded both through unetbooti and through dd. No difference. Boot-menu sees the flash drive.
In the BIOS, I set USB priorities above HDD - zero difference. How to be?
PS There is no drive. Removed, defective. That is, there is only one option - USB

Answer 1

what would I do:
step 1. burn live usb again in the rufus program under windows (a couple of years ago, unetbootin stubbornly couldn’t cope with creating a bootable USB flash drive with I don’t remember what kind of Linux it was). This program saved me. I never recorded through dd. Now on my laptop windows, ubuntu-mate, android. I installed all operating systems from a flash drive recorded by rufus.
step 2. a second after pressing the power button of the laptop, press f12 repeatedly (on my acer-e f12, other manufacturers may have a different button). In this case, we must get to the menu with all bootloaders on all drives. If the flash drive is not listed in this menu, the flash drive is not written correctly.