boot an alternate kernel

[Ralf Mardorf]

On Thu, 2022-05-26 at 20:31 +0200, Ralf Mardorf via ubuntu-users wrote:
> On Thu, 26 May 2022 20:23:41 +0200, Ralf Mardorf wrote:
> > > Should it work from only one partition or should it work from either
> > > partition?  
> 
> > You probably installed GRUB 2 times. One is the GRUB used by the boot order
> > of your BIOS settings, the other GRUB is unused. You are likely booted
> > to the wrong install and generating the menu.cfg for the unused GRUB.
> 
> In your case it's likely possible that you can use both GRUB
> bootloaders, but you need select one bootloader by the BIOS' setting
> boot order and you need to update the menu.cfg of both GRUB.

IOW actually you need to update only one menu.cfg. The one of the disk
that is selected by the 1st drive of the BIOS boot order. To do this you
need to run the install of this GRUB, not the install of the unused
GRUB. It's not possible to do this from the other install by a
systemd-nspawn or by a "regular" chroot. Actually you could copy the
menu.cfg from one to the other install.