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Re: Booting from a 4k partition possible with legacy BIOS?

From: Andrei Borzenkov
Subject: Re: Booting from a 4k partition possible with legacy BIOS?
Date: Sat, 19 Sep 2015 19:33:31 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0

19.09.2015 17:33, Adalbert Hanßen пишет:

I have a Lenovo T400 laptop which uses a legacy BIOS which can not boot
from an UEFI partition. I want to install Ubuntu on a 3TB external USB
disk to make it a rescue disk  to store images from the laptop builtin
disk on it.

The external USB disk has 4k sectors instead of the usual 512k ones
because it is a 3TiB-Disk which exceeds the 2TiB-Limit addressable with

Originally the disk had a traditional "msdos" partition table, hower
with 4k sectors. I could not boot Ubuntu from it.

Then I learned that one can install Ubuntu on a GPT formatted disk, if a
dedicated boot sector of appropriate size is provided. I changed the
partition scheme of the disk to GPT, I installed Ubuntu and configured
GRUB2 on this disk accordingly (no error messages during ths process).
However, I could not boot from the external disk.

I started a thread in the German ubuntu wiki,
it extends through three pages to

One idea from this discussion is, to configure GRUB2 on the builtin
drive such that it would also offer to boot from the external USB disk.
However it looks like Grub does not recognize the USB disk in the phase
of the GRUB menu. I entered *ls* there which returns:

*(hd0) (hd0,msdos5) (hd0,msdos1)*

   * Do you have any idea, how to overcome this situation?

   * Is GRUB2 started from the traditional 512k-sector builtin diskable
     to detect an installed Ubuntu  on the external USB drive once it has
     come up to the moment, when the GRUB menu becomes accessible?

   * Will it be able to boot Ubuntu installed on the external disk
     (directly, not using the GRUB2 installed there whicht might not work
     because my BIOS is not "GPT aware")?

GRUB is using BIOS to access disks. If your BIOS does not see (or expose) disks, GRUB cannot use them.

You could try "nativedisk" command in GRUB command line. It replaces biosdisk driver with direct ATA/AHCI/USB drivers. It /may/ work but is much less tested and has some limitations (like not being able to chainload).

This problem seems to become more and more severe since at my last visit
of a computer shop, all external storage disks in the TiB range were
4k-sector ones. The vendor told me that one can distinguish them from
512k-onnes, if they do NOT mention to be Windos XP compatible. This is
the criterion to distinguish Advanced partition disks from the others,
they told.

Kind regards

Adalbert Hanßen

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