Booting from a 4k partition possible with legacy BIOS?

[Adalbert Hanßen]

Hello,

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 512k-sectors.

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 https://forum.ubuntuusers.de/topic/kein-booten-von-einer-externen-3tb-festplatte-/, it extends through three pages to https://forum.ubuntuusers.de/topic/kein-booten-von-einer-externen-3tb-festplatte-/3/.

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")?

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