Now we are already wasting time, but only so that it doesn't happen to you again next time. You updated the bios. After updating the bios the configuration returns to the default state. To leave it as it was before updating you must review all the settings. If the boot mode changed in the bios and you didn't return it to its previous settings that was the reason that USB stick wouldn't boot. If you changed that configuration now you could start the system again with that pendrive, try it.
Find out on Google about this.
Good luck.
You're more focused on pretending you were right (despite not being right) than reading what I'm actually saying so I'm setting the record straight in case someone else has this happen in the future. Since you live in this forum it might be useful in a future case. Again: I did not change bios mode to legacy or uefi, Gigabyte doesn't even automatically reset the bios settings for this MB after an update, they ask you to do it yourself.. which I did AFTER I had tried everything else in order to see if that helped in booting the drive (it didn't). It's not like I changed that much from stock settings anyways. I still have the old installation by the way (like I said earlier, but you aren't actually reading what I'm saying), I reinstalled OMV6 in a different one from the same pack of drives. MB has same settings, reinstalled USB boots normally, the USB drive with the old installation still doesn't. USB with the old installation seemingly has all the files but the bios update might have overwritten something in it (reinstalled drive is recognized by mobo as "debian", old installation drive just shows the brand name). My takeaway is that I should've disconnected all other usb drives from mb before the update, which in retrospective sounds like common sense but Gigabyte did not reference this in their documentation, their Q-Flash tool did not warn me about it either. For reference, I did fish out the settings to change to legacy mode (gigabyte calls it CSM on this mobo), it didn't help in booting the old installation either.