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Honestly the Fusion Drive is good for some people that need a little increase in speed and want the huge storage that you can't get in an SSD. I recommend using a PCIE SSD and having an external HDD for backups when I need them. The other thing with fusion drives are that your basically relying on 2 drives to work together and you know what happens when 1 stops.

If either one of the SSD and HD fails your OS is toast but can't you just at that point decouple the drives and then reinstall OSX on the working one?

So the more likely scenario is the 1 TB HD fails sometime down the road(past your apple care). At that point you decouple the drives, install OSX on the 128 GB portion and then setup external drive. If the SSD portion fails you do the same process. Obviously you are now stuck with a HD based iMac which is slow but it's better than dropping the money to get it fixed on a old iMac.
 
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