Well, https://kb.netapp.com/support/index?page=content&id=3012152&locale=en_US&access=s suggests as one more reason mismatched pools; if everything listed in this article is OK, I give up - call NetApp support J

From: Edward Rolison [mailto:ed.rolison@gmail.com]
Sent: Thursday, June 11, 2015 2:26 PM
To: Borzenkov, Andrei
Cc: toasters@teaparty.net
Subject: Re: Disk auto assign

New disks are all listed in 'disk show -n'. I think that means they're not assigned. (And a disk I've deliberately unassigned is also not being reassigned). 

On 11 June 2015 at 12:12, Borzenkov, Andrei <andrei.borzenkov@ts.fujitsu.com> wrote:

Are you sure new disks are unowned? It is not uncommon to get repurposed hardware.

From: Edward Rolison [mailto:ed.rolison@gmail.com]
Sent: Thursday, June 11, 2015 2:09 PM
To: Borzenkov, Andrei
Cc: toasters@teaparty.net
Subject: Re: Disk auto assign

Sorry, should have said - also did this with the spares. I can see no drives on 'head1' sysconfig -r that are 'wrong' and likewise on 'head2'. 

Checked this with grepping 'partner' and ensuring they're all the right pair of controllers. (On both sides)

On 11 June 2015 at 11:56, Borzenkov, Andrei <andrei.borzenkov@ts.fujitsu.com> wrote:

If you just performed “replace” without un-/re-assigning drives that became spare, this is expected. Spares still belong to controller and will block auto assignment.

From: toasters-bounces@teaparty.net [mailto:toasters-bounces@teaparty.net] On Behalf Of Edward Rolison
Sent: Thursday, June 11, 2015 1:09 PM
To: toasters@teaparty.net
Subject: Disk auto assign

I've just added some new shelves to a (7-mode) filer pair, bring them up to two stacks of 10 shelves.

I'm trying to get 'autoassign' working, and it isn't.

I have:
- swapped drives such that each 'head' has only disks from one 'stack'. 

Head 1 has 0a/5b

Head 2 has 5a/5d

I've had to manually trigger a small number of 'replace' operations. 
And then switched on autoassign in the options.

I _think_ that should be it, but ... there's something I'm missing, because autoassign isn't working. 

Does anyone have suggestions for why this might be, or what else I could check? (I'm wondering if there's a misassigned but 'failed' drive that otherwise isn't showing up?)