Re: USB3 3TB HDD boot

[Bruce Dubbs]

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
> > > need:
> > >
> > > 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