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Re: Unable to boot BIOS/GPT system.

I'd expect that Grub won't be able to reliably work with that setup -
partition 3 goes past the 2TB mark, so the BIOS won't be able to map
it properly. Don't forget, when grub is reading the disk all it can
rely on are BIOS calls. Add yourself a small-ish /boot partition first
and you may be OK.

Thanks, indeed this was the problem - after adding a boot partition
the HDD now boots on BIOS-only machines. Unfortunately, it does not
boot on UEFI machines. Seems that I have to start experimenting with
hybrid UEFI+BIOS boot...