Backup and Restor MBR

[Mike McCarty]

Aaron Konstam wrote:
> On Mon, 2006-12-11 at 16:23 -0600, Mike McCarty wrote:
> 


>>>>
>>>>The MBR is the Master Boot Record, which is 512 bytes long
> 
> 
> This is both true and not true. It is true that the whole 512 bytes is
> called the MBR but if we mean what gnome-install updates as the MBR only
> 446 bytes are in this MBR.

The MBR has a structure to it. No question. The *code* portion (which
is unimportant, actually, as it contains no useful information which
cannot easily be gotten from other sources) is 446 bytes long.

>>>>The MBR comprises three parts
>>>>
>>>>1. Code
>>>>2. Partition Table
>>>>3. Boot Record Marker (AA 55)
>>>>
>>>>No part of the MBR may be read or written without reading
>>>>or writing all of it. Discs are, after all, BLOCK DEVICES
>>>>which cannot transfer any amount less than a block (sector).
> 
> It is true thast disks are block devices but are you saying:
> dd if=/dev/hda of=/path/filename bs=446 count=1 will not put 446 bytes
> into the file. And what does fdisk do when you do a write. What part of
> the 512 byte block does it change.

Yes, it puts 446 bytes into the file, but it puts 512 bytes
on the disc.

When fdisk writes the MBR, it writes 512 bytes. One cannot write
less than one full sector to disc.

[snip]

>>>>Saving the MBR should be done in its entirety. Whether one wants
>>>>to preserve the PT portion of the MBR when rewriting it on disc
>>>>later is a decision to be made at that time. Not saving the original
>>>>PT portion of the MBR is folly.
>>>>
> 
> Why exactly is it folly?

If anything needs to be "saved" it is the PT portion of the MBR.
The code portion is uninteresting and not particularly useful
to save, since it can be gotten from a number of sources.
The *important* information is contained in the PT. The stuff
we can't get from another source easily.

I'm not recommending only to save the PT portion. It's best
to save it all. But if I had to choose between the code
portion and the PT portion of the MBR, I'd save the PT part
no question, and leave the code.

>>When somebody states "No" four times in a row, and then gives incorrect
>>data and actually bad advice, he needs to have his wrist slapped. I
>>wasn't correcting *words*. I was correcting bad information and bad
>>advice.
>>
>>Recommending not saving the entire MBR is BAD ADVICE, which he
>>based on INCORRECT INFORMATION.
> 
> What incorrect information is it based on?

See right here below

>>Advising someone not to back up the entire MBR, based on the
>>misinformation that the PT is not part of it, is foolish.

Mike
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