Unexpected bios behavior??? due to connection of an external drive.

[Chris Murphy]

On Tue, Mar 17, 2015 at 6:08 PM, jd1008 <jd1008 at gmail.com> wrote:
>
>
> On 03/17/2015 05:58 PM, Chris Murphy wrote:
>>
>> The solution is to go to firmware setup and define a different drive
>> as primary. Or zero the first 440 bytes of the external drive e.g. dd
>> if=/dev/zero of=/dev/sdX bs=440 count=1
>
> Interesting.
> I used fdisk.

It may have had something else partition it before. What do you get for

dd if=/dev/sdX count=1 2>/dev/null | hexdump -C


> I guess fdisk does not clear those first 440 bytes?

No. It's somewhat archaic, from the era when bootloader and partitions
were married. Hence the MBR (LBA 0) is sometimes used to mean the
partition map portion, and other times it refers to the bootloader
code in the first 440 bytes of that same sector.


> I am a bit concerned about dd'ing 440 zeros from /dev/zero
> into this drive, as it has about 0.5 TB of data on it, and I have
> no room (yet) to back it up to :)

Well the partition map data doesn't start until offset 446, so as long
as you make sure bs=440 *and* count=1 it's not a problem. But you can
also just redirect the firmware to another drive to avoid the problem.


-- 
Chris Murphy