I haven't received my 2013 iMac which after much deliberation I went with a 1TB Fusion Drive. I had considered a 256GB SSD. I am now thinking after something I read today of sending the 1TB Fusion back, getting a 256GB SSD model, adding an external 2TB USB 3.0 drive and merging the SSD and USB 3.0 into a Fusion volume. From what I understand, this can be done.