installed on USB -- must change port after update (but no ports left)

[Carnë Draug]

Hi everyone

I have Debian installed with GRUB2 on a USB stick. However, it seems
that after an update (it runs grub-mkconfig -o /boot/grub/grub.cfg), I
can no longer boot from the USB port. When the BIOS lists the boot
options, it doesn't even show my USB stick anymore.

I'm of course not sure if this is the cause of my problems but I don't
know what else can be. I have this PC only temporarily that has 2 USB
ports. It booted fine before, no problem. One day, after an update, I
could no longer boot. I tried the other USB port which was ok so I
didn't gave it anymore thought (but the other USB port still works.
After booting, I can connect external drives to it for example). This
morning, after another update, I also can't boot from this second USB
port. I tried the 1st again bt still nothing. So the PC is useless for
me now. I can't boot on my system anymore.

I was wondering if it's possible that when creating a new grub conf
file, it somehow blacklists that port, identifies it as an HD,
something like this. How can I solve this? Or prove that the issue if
not with GRUB at all? Any help would be very welcome.

Thanks in advance,
Carnë