Comment 10 for bug 1284196

I've tried to reproduce Alexander's report of an EFI-mode GRUB failure without success:

I first created a VirtualBox BIOS-mode Ubuntu 15.10 installation in a 4TiB virtual disk. To "stack the deck" in favor of a failure, I created a separate /boot partition at the END of the disk, ensuring that kernels, initrd files, and GRUB's configuration files and modules all resided over the 2TiB mark. As expected, this failed to boot in BIOS mode, as in my initial bug report, with one exception: The error message was "unknown filesystem," not "attempt to read or write outside of disk `hd0'."

I then switched to EFI mode and used rEFInd to boot the computer, which worked fine; clearly the VirtualBox EFI has no problems reading beyond the 2TiB mark. (rEFInd relies on EFI system calls to read all its files.)

With the system booted, I installed the EFI version of GRUB, including "sudo grub-install" and "sudo update-grub." I then rebooted, and GRUB was able to boot the computer.

Thus, I think we can rule out the possibility of GRUB itself, at least in EFI mode, having problems reading beyond the 2TiB mark. I see two possible causes of the problem that Alexander reports:

* Alexander may have mistakenly installed in BIOS mode rather than in EFI mode. This is notoriously easy to do, even for experienced users.
* The EFI in Alexander's Supermicro computer may have a bug akin to the one in VirtualBox's "BIOS." This is a more disturbing possibility, because it means that, if such bugs are common, we may be seeing more of this problem in the future, as disk sizes increase, even on EFI-based computers.