I'm trying to work out the best way to configure my drives so that I get the most speed out of them. My Samsung 850 EVO 500GB contains macOS 10.12 and all my applications with little User data, and is attached to an OWC Accelsior S 2.5" PCI4 card giving me SATA-III speed (bootable). My main User data (1.5TB) is on a Seagate 3TB HDD, plugged into Bay1, and I have a 2TB and a 3TB HDD in two of the other Bays, plus a 1TB HDD in the spare Optical Bay. My 3TB User drive is the weak link as far as speed goes. Having the SSD in the PCIe card gives me exceptionally fast boot and application launches. But I want to get another SDD (I'm looking at the Crucial MX300 575GB - anyone got a bad experience with these?) to create a Fusion Drive for my User 3TB so that I can get some more speed when I open/save/copy/move files etc. I can't afford to buy a huge >2TB SSD. So I need to find out the best way to configure my drives. 1. Is it possible to create a Fusion Drive (FD) with a SSD in the Accelsior S PCIe card and a HDD in one of the normal direct connect Bays, or do both the SSD and the HDD have to be in the Bays? 2. Would my system benefit more from having the SSD FD (containing my User data) in the Accelsior PCIe card, or am I best to have my boot SSD (containing the OS and all my Apps) in the PCIe card? More simply put, would my boot OS or my User data FD benefit more from SATA III? I'm noticing a slight speed increase having the boot SSD in the PCIe versus having it in one of the direct connect Bays. But I seldom see read/save bursts over 200MB a second when opening apps so don't know if it is really making the best use or needs SATA-III capability. Opening/saving/copying files is all done on my User HDD, and I often see read/write speeds up to 120MB a second, which is the best I can expect from a mechanical HDD on the SATA-II direct connect bays. I'm just waiting for an Intel Xeon X5680 3.33GHz 6-Core CPU to arrive so I can upgrade from my 2.66GHz Quad core - that should give me a handy boost. Getting a dual CPU tray is just too expensive (unless someone knows of a cheap place to get them). And I have a USB powered Mini PCIe card for my WiFI ac/BT 4 on the way too so I'll have Continuity/Handsoff. Any advice on what else I can do to upgrade my cMP is welcomed.