Re: Clarification of what grub-install does, versus grub-mkconfig/update-grub

[Pascal Hambourg]

Le 22/11/2019 à 23:07, Chris Green a écrit :
> On Fri, Nov 22, 2019 at 10:40:18PM +0100, Pascal Hambourg wrote:
> > No, there is no default location for the boot image and core image. Anyway
> > "first hard disk" is meaningless. First for what ? The BIOS ? Linux ?
> So how does it decide?  ... or at least how does, for example, an
> Ubuntu install decide?

I don't know how the Ubuntu installer works. The Debian installer asks 
the user to choose.

> Yes, I see that, one can install them on every drive but which gets
> run at boot time then?

The one on the disk booted by the BIOS according to its boot preference 
order.

> > > What I still don't quite understand is what happens with (as I have)
> > > more than one OS installed.  Presumably only one grub.cfg gets used
> > > even though there is more than one. How does the initial boot process
> > > (starting with MBR etc.) and created by grub-install decide which
> > > /boot it should use (and is it easy to change which /boot it goes to?).
> >
> > The BIOS boots one drive. Its MBR contains a GRUB boot image which loads and
> > runs the core image of the same installation, which loads modules and
> > configuration files from the same installation.
> But someone said it doesn't matter which drive is marked as the boot
> drive,

What do mean by "marked as the boot drive" ?
As I already wrote, there is no disk boot flag in the MBR, only primary 
partition boot flags. The BIOS should not care about the boot partition 
flag, but some non compliant implementations consider a disk for booting 
only if one of its partitions has the boot flag set. If your PC has such 
BIOS implementation, you can set the boot flag to a primary partition on 
the boot disk.

> how does the BIOS decide which drive to boot from?

From the boot order defined in the BIOS settings.