[ILUG] harddisk swap

[Timothy Murphy]

On Friday 30 April 2004 07:15, Enda wrote:

> > > Where the drive class is the same, ie, SCSI / IDE etc, the device
> > > labels are the same, and with partition schemes been suggested
> > > identially out of
> > > the box, the probability of swapping MBR's and them working is very
> > > very high.
> >
> > Have you ever done this?
>
> Yes. Its especially useful in scenario's like the posters, where you dont
> have physical immediate acccess to the machine, and when you do get access,
> you dont want to be standing around pointing and clicking on gui installs,
> and you'd rather not have to config that quirky lan card driver / xfree86
> config again. Just copy the files, and twig the boot environment.

What exactly do you mean by "twig the boot environment"?
If you mean, run LILO or grub-install
then this over-writes the MBR, so any previous copy was a waste of time.

I would normally defer to you ILUG gurus, 
but in this case I think you are completely wrong,
or else expressing yourselves badly. In my view,
the only way in which a copy of the MBR from one disk 
could work on another disk
would be if the entire disk had been copied byte-by-byte, eg with dd.

The following is my understanding.
The boot loader in the MBR does not know anything about the file-system,
so in order to access the grub_1.5 stage it must contain
the exact physical location of this file.
There is absolutely no reason why the file should be in the same location
on different disks,
unless as I said the whole disk had been dd-ed,
which was not the case for the OP,
since he/she explicitly said that cp had been used.

In general it is foolish advice, IMHO, to suggest that
the MBR from one disk should be copied to another,
since it could perfectly well get people (particularly newbies)
into a very confused state.

The proper advice is that having copied one disk to another,
in whatever way, if they want the second disk to be the new master
then they should ensure that /etc/fstab and /etc/grub.conf or lilo.conf 
are correct, and run LILO or grub, pointing to the new conf file.

As I mentioned, all this is explained very clearly
in the Hard-Disk-Upgrade HOWTO
(except that last time I looked this was oriented entirely
towards LILO, and didn't mention grub).



-- 
Timothy Murphy  
e-mail (<80k only): tim /at/ birdsnest.maths.tcd.ie
tel: +353-86-2336090, +353-1-2842366
s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland