From owner-freebsd-hardware@FreeBSD.ORG  Mon Nov  3 04:46:36 2014
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Date: Sun, 2 Nov 2014 21:25:37 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: John <freebsd-lists@potato.growveg.org>
Subject: Re: gptboot: invalid backup GPT header
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On Mon, 3 Nov 2014, John wrote:

> On Sun, Nov 02, 2014 at 07:15:59AM -0700, Warren Block wrote:
>> Clearing the last blocks will remove the warning, at least if the
>> assumption is correct.  ZFS is not supposed to be using the last
>> portion of the disk(*) to make replacing disks of slightly different
>> sizes easier.  Still, I would back up either the whole array, the
>> whole disk, or at least the last 33 blocks of mfid1 first.
>
> Thanks for replying. So, in short - is my data on the array in any
> danger, or is this just a cosmetic issue?

If the guess is correct, it's just cosmetic.  Unless someone panics at 
the warning and tries to repair it.