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Re: USB3 3TB HDD boot

From: Bruce Dubbs
Subject: Re: USB3 3TB HDD boot
Date: Fri, 15 Mar 2013 11:25:23 -0500
User-agent: Mozilla/5.0 (X11; Linux i686; rv:13.0) Gecko/20120628 Firefox/13.0.1 SeaMonkey/2.10.1

Chris Murphy wrote:

On Mar 15, 2013, at 8:51 AM, Lennart Sorensen
<address@hidden> wrote:

On Thu, Mar 14, 2013 at 08:56:18PM -0600, Chris Murphy wrote:
Right, for a 4Kn drive, to read LBA 1 and get the GPT header I'd

sudo dd if=/dev/disk3 skip=8 count=1 2>/dev/null | hexdump -C

Wouldn't using 'bs=4096' make things simpler and more obvious?

How would it?

These structures are still predicated on a 512 byte block. The MBR is
always 512 bytes, but on a 4096/4096 4Kn drive, LBA 0 is 4096 bytes.
So if I don't want to see 3584 bytes of useless garbage, I can't set
the dd block size to 4096, or I get extra information.

If I want to see just the GPT header, which while only ~92 bytes, by
spec it gets its own sector, there's far less superfluous information
using a bs of 512 bytes than 4096 bytes.

There is more than one way to do things.

sudo dd if=/dev/disk3 bs=4096 skip=1 count=1 | cut -c 1-92 | hexdump -C

is one way that matches the physical drive.

The first mentioned way works too, but doesn't reflect as well what is really going on.

  -- Bruce

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