I purchased a 2TB Fusion Late 2015 27" several months back. In hind site I should of gotten the 256GB storage option for my use. On my old iMac 2007 I had about 180GB of data used but after cleaning it up when I got the new machine I realized I had a lot of garbage and duplicates. Now I am only using about 70GB of hard disk space. It seemed like such a waste for me not to use the extra storage so I thought I would put the spinner to use as Time Machine and remove one of my external hard drives. I do have a Crucial 275GB MX300 in a inaTek USB 3.0 enclosure that I have setup as an additional boot drive with OSX and some games I seldom play. I do not recommend un fusing the drives. It does not run any faster then fused. I get the same geek bench scores, I get the same read and write scores, it basically operates the same as before except I did not have enough data to take advantage of the Fusion. If you have over 100GB of data it only makes sense to be FUSION. Plus it's a pain to switch back and forth. After that is all said I am impressed with the read and writes on the spinner, 170's read and writes. I labeled the internal 2TB spinner drive iMac Time Machine. Crucial is the exteranal 275SSD MX300 and Sys-Volume is my internal 128GB SSD. The DiskMark is an app in the app store I loaded and it closely follows the BlackMagic app. I usually see higher reads with BlackMajic on the internal SSD in the 1600-1800 but never on the DiskMark app, it comes in below 1400. I like DiskMark for its one time read and write and log the results.