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...
