My Fusion set up crashed and it took me two days to get it back up and running before I remembered something I ran into a couple years ago. [For Reference Top slot means top when Mini is upside down. Bottom slot means the buried slot] I absolutely could not get a working Fusion setup with a 750GB 7200RPM drive installed in the Mini. I tried a Seagate MomentusXT 750GB "hybrid" (ST750LX003) and a Western Digital Black 750GB (WDBLHJ7500ANC). [FYI: The 750GB drives will absolutely install by themselves just fine, its only in a Fusion environment do they cause problems.] With the HDD in the bottom tray and SSD in top, the Fusion volume would create but OS X could not write to the volume. So it would usually fail at trying to create Recovery Partition. But even if you could trick the system to not make a Recovery Partition, it would just crash at the next thing. Time Machine also would not work. With the SSD in the bottom tray and HDD in the top tray, the SSD became unresponsive. When trying to do any kind of "erase" or "partition" it would fail for not being able to unmount the drive and/or volume. This was when trying to make the coreStorage volume....before making the "Macintosh HD" volume. diskutil cs create FUSION disk0 disk1 would fail for not being able to unmount disk0. So in this combination I could not even make a Fusion volume. Putting either a 500GB or 1TB (both 5400RPM for me) in the bottom slot and any SSD in the upper slot creates a Fusion drive with recovery partition with zero problems. So I don't know if it's the 7200 RPM that's the culprit or the 750GB aspect. But I would suspect the 750GB as that was a capacity not offered with Apple computers that had Fusion available. Mavericks CAN have problems making a Recovery Partition on an homemade Fusion setup. But Yosemite and El Capitan are fine with it. Once you blow out your Recovery Partition you will have to use Internet Recovery, which for me meant it tried to do Mavericks (in Jan 2016). So what I ended up doing was installing Mavericks to a USB 3.0 flash drive (just select the USB drive as the install destination), booted the Mini off that and then download El Capitan and made a bootable flash drive that way. Then installed El Capitan to the Fusion drive. Good to keep that USB stick around too Google-Fu OS X Fusion trouble 750GB, Fusion SSD slot. Using MomentusXT 750GB.