[CentOS] Analyzing the MBR

Thu Jun 5 17:47:16 UTC 2014
Timothy Murphy <gayleard at eircom.net>

David Both wrote:
> The dd command shows you **exactly** what is in the MBR and, if you want,
> the following sectors. But the following sectors are not particularly
> relevant to boot. THe MBR contains the boot record and the partition
> table. There is not room for anything else. But your problem is not with
> the MBR so the solution does not lie there.

Is this strictly true - that only the MBR is read at boot-time?
I have saved and re-inserted the MBR on occasions
(not in the present case)
and it did not seem to be sufficient.
I notice for example that gparted leaves quite a lot more space
at the beginning of the disk, as does Windows.

> Although you have already reinstalled, you might have recovered by
> changing the boot order of the hard disks in the BIOS configuration for
> your computer.

Well, that is more or less what I said -
I only realized that was the problem after trying many other solutions.
In self-defence I would say that I read half-a-dozen articles on the web
telling you what to do if grub fails,
and none of them mentioned the possibility that the hard disk order
might have been changed - in my case by running the system 
with one disk removed, and then with the disk back in place.

Timothy Murphy  
e-mail: gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin 2, Ireland