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Re: dd effect on cloning of iso file to usb stick

From: Eric Blake
Subject: Re: dd effect on cloning of iso file to usb stick
Date: Fri, 03 Oct 2014 07:04:51 -0600
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On 10/03/2014 04:56 AM, jb wrote:

>>> # fdisk -l /dev/sdb
>>> ...
>>> Device     Boot Start     End Sectors  Size Id Type
>>> /dev/sdb1  *        0 1171455 1171456  572M  0 Empty
>>> /dev/sdb2         252   63739   63488   31M ef EFI (FAT-12/16/32)
>> ... 

> W/r to that interpretation, in a dos (MBR) partitioning scheme, the hard
> disk can contain at most four primary partitions, or alternatively
> three primary partitions and an extended partition.
> The only way to have embedded (logical) partitions is thru dedication of
> one primary partition to an extended partition mechanism.
> What the fdisk shows here, at least start/end sectorwise, is that sdb2
> partition is physically embedded in sdb1 partition.

Yep. An unusual layout to be sure, but completely valid (as evidenced by
the fact that your kernel was able to mount both partitions).

> So, how to interpret sdb1 and sdb2 partitions (primary, extended,
> logical) ?

sdb1 is ALWAYS the first primary partition, and sdb2 is ALWAYS the
second primary partition.  It is possible to have an MBR partition table
that specifies /dev/sdb2 while leaving /dev/sdb1 uninitialized, although
that is also unusual.

> Is this a case of extended partition mechanism ?

No, it is a case of someone intentionally overlapping two primary
partitions.  Weird, but nothing inherently wrong with it.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library

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