On Wed, 8 Jul 2015, jtroan@jt-sw.com wrote:

I'm also thinking about adding a fourth drive and try to config it as a 
hot-spare, giving me some extra margin for failure.

I would recommend you turn your raid5 into raid6 instead of having a 
raid5+spare. This can be done with a fairly simple command, without 
downtime.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se