Re: Booting from alternate hard drive

[Pascal Hambourg]

Le 02/05/2018 à 23:16, Chris Jones a écrit :
> I have a laptop with two hard drives and with debian/stable as the main
> OS - that's where I keep my grub environment.
>
> This main debian/stable OS resides on disk SDA together with the other
> linux installs I occasionally boot into.
>
> Having run out of space on SDA I installed another flavor of linux
> (Deepin) on the laptop's second drive (SDB)
>
> As per "fdisk -l" identifies it as /dev/sdb10.
>
> I was able to mount this partition from the debian/stable system and the
> contents are indeed as intended.
>
> In order to boot this new system I proceeded to add the following to my
> /etc/grub.d/40_custom config file:
>
> menuentry " Deepin 15.5 - 64bit, with Linux 4.9.0-deepin13-amd64 (on 
> /dev/sdb10)" {
>    insmod gzio
>    insmod part_msdos
>    insmod ext2
>    set root='(hd1,msdos10)'
>    search --no-floppy --fs-uuid --set=root 60e5f04d-419c-4ceb-a0d2-09fcf36e30c3
>    linux        /boot/vmlinuz-4.9.0-deepin13-amd64 
> root=UUID=60e5f04d-419c-4ceb-a0d2-09fcf36e30c3 ro reboot=pci
> }

Why didn't you just let os-prober detect the other OS ?

> I ran update-grub and restarted the box and the new entry was indeed
> visible in grub's chooser/menu.
>
> Unfortunately, when I select the Deepin entry I end up with some kind of
> kernel panic.

"Some kind" does not provide much information. There are many kinds of 
kernel panic. I suspect that the kernel panic is caused by the lack of a 
"initrd" command in the menu entry. root=UUID= requires an initrd or 
initramfs because the kernel itself does not know anything about 
filesystem UUIDs (only about partition UUIDs, PARTUUID). An initramfs 
can be embedded in the kernel image, but most often it is in a separate 
file named initrd*.

> As far as I understand it... grub is installed in the MBR of the two
> drives (I ran dpkg-reconfigure grub on this debian sysstem and both
> devices were listed).

Irrelevant. The GRUB instance installed on the second drive is not used. 
You're not booting from it, you're just loading the kernel from it.