Strange booting problem

[Rick Stevens]

On 07/01/2015 02:39 PM, jd1008 wrote:
>
>
> On 07/01/2015 03:14 PM, Matthew Woehlke wrote:
>> On 2015-06-30 19:01, jd1008 wrote:
>>> So, how can I proceed with a brand new drive,
>>> dd /dev/zero into the first ... say 4K bytes, partition
>>> it with fdisk, do not mark any partition bootable, so
>>> that bios will skip over it ?
>> Don't know why no one's mentioned this, but... you could always just
>> install an actual bootloader on the drive that boots from the device
>> from which you really want to boot. (I think you can do this with
>> grub...)
>>
>> Of course, plugging that drive into any other computer might make for an
>> interesting experience :-).
>>
> I am sorry - but ...
> the design and implementation of the traditional
> (msdos) scheme and ( from what I understand so far
> from all the respondents), even gpt, effectively render
> the disk to have a signature which BIOS interprets
> as a valid partition table AND as bootable, and thus
> hangs there looking for what does not exist.
>
> Why the design mixed 2 different things into 1, I have
> no idea. But AFAIAC, it sucks and blows atthe same time.
>
> Theoretically, supose I want my PC to have 2 identical drives,
> partitioned identically, both bootable.
> Say the boot order is cd-rom, drive A, then drive B.
> CD-rom is empty, bios moves on to drive A. Somehow
> drive A's boot code is corrupt (say somehow all nulled).
> PC will never move on to drive B.
>
> So, instead of fixing the issue, we invent new, complex
> schemes that require even more complex SW like VM's,
> LVM's, .... etc etc.. to solve a problem created by a very
> silly error: partitioned means bootable as far as BIOS
> is concerned. At least, that is what I have come to understand
> and experience (when I removed the boot signature bytes).

Simply nulling bytes 510 and 511 of the first sector (getting rid
of the boot signature) should make the BIOS ignore the drive. fdisk
will complain that the partition table is invalid because of the
invalid boot signature, but that's all. The rest of the drive should
be functional and usable--just not bootable.

Tagging a partition as "bootable" only affects Microsoft OSes. The
boot loader itself doesn't care. What that boot loader loads and
hands control to--THAT might care about the bootable flag. The Linux
kernel ignores it.
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- Rick Stevens, Systems Engineer, AllDigital    ricks at alldigital.com -
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