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We have an older iMac (Model A1311, Mid 2011). It has been running Yosemite. We have run into some of the WIFI issues associated with Yosemite.
Is it possible to upgrade to El Capitan but not Sierra?
El Cap has been out longer. Its my wife's computer, its older, and I really don't want to be debugging Sierra until a few more "dot" releases come out.