I created a fusion drive three years ago consisting of an internal 1TB mechanical HD and an internal 250 GB 840 evo SSD in a mid-2011 iMac running Sierra. Total HD space is 1.19 TB with 383 GB of free space remaining. I have purchased a 1 TB 860 evo SSD to replace the mechanical drive. If I break the fusion drive and use just the 1 TB SSD for my system the SSD will be nearly full. The 250 GB SSD will just be a separate drive. My other option is to keep the fusion drive using both SSDs. I'm not sure which is the best way to go although I'm leaning towards keeping the fusion drive. I appreciate your input as to which way to go. What are the pros and cons? Thanks!