I am 100% sure that this thread topic has been covered before, but finding the correct search terminology, is a little hit and miss I find. I have a late 2015 27" iMac that was configured with a 2TB Fusion drive and OS X El Capitan. Pic 1: Screen Shot 2017-02-23 at 10.44.39 by Darren Hughes, on Flickr Pic 2: Screen Shot 2017-02-23 at 10.44.24 by Darren Hughes, on Flickr When I come to install the new OS Sierra using this guide which seems to be the best and most trusted (??) I have found thus far, will I come into any problems with the fusion drive not working as it should i.e. not together. Also, using Time Machine for backup, will that back up the OE apps without have to pay/download them from the App Store? Thank you.