Strange booting problem

[Chris Murphy]

On Tue, Jun 30, 2015 at 5:13 PM, Gordon Messmer
<gordon.messmer at gmail.com> wrote:
> On 06/30/2015 03:18 PM, Chris Murphy wrote:
>>
>> 2. The most likely explanation for the problem, as someone else
>> alluded to, is the USB drive has stale bootloader code on it that
>> points to no where and hangs.
>
>
> One of jd's earlier messages included the boot sector.  It was mostly nul
> bytes.
>
>> The solution is to do one of two things: change the boot order in
>> BIOS; or zero the first 440 bytes of LBA 0 with this:
>> # dd if=/dev/zero of=/dev/sdX bs=440 count=1
>
>
> Why 440?  The boot sector is 446 bytes.

The boot loader code area is variably sized depending on what writes
it out. GRUB's is 446 bytes. But syslinux and variants are 440 bytes,
and the parted code is maybe half that size. So wiping out 440 bytes
is sufficient but there's nothing wrong with wiping out 446 bytes.

Also, "the boot sector" is the same thing as LBA 0, which is the same
thing as MBR, in the present context. A sector is 512 bytes.

-- 
Chris Murphy