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Date: Thu, 29 Sep 2016 04:34:58 +1000 (EST)
From: Ian Smith <smithi@nimnet.asn.au>
To: David Christensen <dpchrist@holgerdanske.com>
cc: freebsd-questions@freebsd.org
Subject: Re: FreeBSD-10.3-RELEASE-i386-memstick.img installer changes contents
 of USB flash drive
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In freebsd-questions Digest, Vol 643, Issue 3, Message: 17
On Tue, 27 Sep 2016 14:19:39 -0700 David Christensen <dpchrist@holgerdanske.com> wrote:
 > On 09/27/2016 08:12 AM, Ian Smith wrote:
 > > Just checking .. did you only access the stick by reading with dd?
 > 
 > Correct.

OK.

 > > Where?
 > 
 > The last block:
 > 
 > # ll
 > /mnt/i3000d/data/dpchrist/iso/freebsd/10.3/FreeBSD-10.3-RELEASE-i386-memstick.img
 > 
 > -rw-r--r-- 1 dpchrist dpchrist 691669504 2016/09/25 11:25:37
 > /mnt/i3000d/data/dpchrist/iso/freebsd/10.3/FreeBSD-10.3-RELEASE-i386-memstick.img
 > 
 > # cmp -l
 > /mnt/i3000d/data/dpchrist/iso/freebsd/10.3/FreeBSD-10.3-RELEASE-i386-memstick.img
 > /dev/sdc
 > 691669010 351 235		<< ~18 bytes into the last 512-byte block 
 > 691669011 117 252
 > 691669012   2 351

 > 691669081 121   0
 > 691669082 235   0
 > 691669083 252   0
 > 691669084 351   0
 > 691669085   0 301
 > 691669086   0 203
 > 691669087   0 127
 > 691669088   0 137
 > 691669089   0 337
 > 691669090   0  24
[..]
 > 691669495   0 227
 > 691669497   0  22
 > 691669498   0 163
 > 691669499   0 362
 > 691669500   0 372
 > 691669501   0 221
 > 691669502   0 230
 > 691669503   0 122
 > 691669504   0 256
 > cmp: EOF on
 > /mnt/i3000d/data/dpchrist/iso/freebsd/10.3/FreeBSD-10.3-RELEASE-i386-memstick.img

Right, so most of the last sector was updated.  I'm no good at octal ..

We discerned recently that for 10.3, i386 memstick is GPT/EFI and amd64 
is still MBR/BSD.  Since only the last sector differs, perhaps something 
has updated the secondary GPT during the installation, assuming that a 
GPT image should already include one - or at least, the space for one?

Possibly this doesn't matter at all, but it does seem 'untidy' at best.

I wonder if a hardware-write-protected GPT memstick would have issues?

cheers, Ian