Hello LinuxQuestions forum and thank you for an informative resource which I have used frequently over this past decade without having need to log in, and consequently forgot my old password and cannot access the old email address that I originally joined under anymore, hence the '2' suffix for the resurrection of my original account here.

My question for this decade is one which I hesitated to think of asking for a while because it may seem that I am being pedantic and fussy over such a small incidental as having to unplug a USB cable from the laptop's port everytime I reboot it. Please understand that no matter how gently I try to be with my hardware, inserting and removing USB connectors to and from USB ports does inevitably add to the physical wear on the mechanism, and so I do like to leave things plugged in between reboots if at all possible. I realise I am being fussy, so please ignore the rest of this rant if you find my attitude unacceptable to your understanding. Thank you for reading.

Yesterday I finished a good, hard week's work building a nice new operating system using Qubes-3.2 booting from an external USB-3.0 HDD, and I have another external USB-3.0 HDD (no boot flag set) containing freshly organised copies of all my data for the past 30 years. My Toshiba Satellite i7 model 4??? (2 yo) has 2 USB-3 slots in which both of these two USB drives: 'System'(bootable) and 'Data' (non-bootable), are normally plugged into when I am working. The system uses the old-fashioned MBR/BIOS boot system rather than the new GPT system which I am not yet fully conversant with.

On reboot, I need to unplug the 'Data' USB drive in order for the BIOS to find GRUB on the 'System' USB drive and commence the Qubes Xen boot process. If I leave the 'Data' USB drive plugged in when rebooting, all I get is a black screen with a flashing underscore cursor in the top left corner, and the boot proceeds no further.

Naturally I have tried swapping the USB drives into alternate ports with the same disappointment either way, and this is not a matter of life and death in the immediate future, but as explained above, it will reduce my beloved Toshiba's life expectancy if I persist with this twin USB drive system, requiring the removal of the 'Data' drive on every reboot. I had hoped that my machine could determine for herself which one of the twin USB drives was bootable and which one was not, but alas, she doesn't seem to be able to do what I had hoped.

Now, onto the question.

Has anyone else managed to find a way to boot GRUB from one USB HDD when there is another USB HDD plugged into another USB port?

If I could possibly install another GRUB on the 'Data' USB drive and set the boot flag, and then point THAT GRUB to the UUID of the 'System' USB drive, I guess it might work in theory, but as it would take a lot of trials and more than that many errors, I thought it best to start this new week by re-registering here and inquiring whether anyone else in the World has succeeded in booting from 1 of 2 USB HDDs in practice.

I expect quite a wait before any solutions to this question because this is not the sort of thing people tend to do with their hardware all that often, so please do not feel compelled to reply if you are not entirely familiar with this quirky dilemma. Thank You.

1. You got USB HDD's or internal HDD plugged into USB's ports?
2. Grub and boot order, USB ports, two or more plugged in at same time, BIOS getting confused to which one to pick, one has boot set to it, another does not.

( the MBR of the one that does not have boot flag, it is clean and never been written to in the manner of a boot sector or grub etc.. )

Figure out your USB Port order then try putting your BOOT USB HDD in to your first USB PORT then whatever in the other ones.

yes USB Ports have an order to them, at least mine does. that seems a logical idea to try first.

UUID vs old school /dev/sda too should then play a part in it because of initiation order of your USB Ports, whatever gets it first gets the /dev/sdb then sdc etc.. so your port order too plays a part in that if you are not using UUID.

As mentioned, using UUID or PARTUUID is the best solution.
BTW, GPT and UEFI are not tied together. You can use GPT partition table without UEFI. Some crappy operating systems will not accept it, but Linux in general boots fine.

After all that, it all came down to one stroke on the F1 key at boot, which for some strange reason points the Toshiba in the right direction to knowing that the USB drive with the boot flag set is the one to boot from. Turning Japanese I think I'm turning Japanese I really think so :-) ...