So, I've bought an used late 2012 mac mini. It came with the original 500gb mechanical drive and an aftermarket samsung 840 evo, 250gb.

Yosemite was installed, and the drives seen as fusion drive. The system was affected by spare errors (attempts to continuously recover keychains, etc..).

I performed an NVRAM reset and splitted the two drives via the usual terminal commands, and installed Yosemite on the newly formatted 840 evo, Internet installation. Installation succeeded, and system rebooted, BUT with no luck: it entered in the efi boot menu. Rebooting with command pressed (boot menu), the machine doesn't even see the installation partition on the ssd.

I tried to install osx on the machanical drive, but again with no luck: disk utility shows it as locked, impossible to unmount.

So my questions are essentially three:

  1. Why my system installs osx on the ssd with no errors, but doesn't see the installation?

  2. Why the mechanical drive is locked?

  3. It is possible that my mini hardware is failed?

  • You can try with only SSD connected and remove your mechanical drive. – Adnan May 16 '15 at 13:58
  • This is feasible. But I'd prefer to not open the Mini and mess with commectors, if possible: I would lose the chance of returning the mac mini to previous owner. – MadHatter May 16 '15 at 14:01
  • Ok. System Preferences > Startup Disk. Did you check which disk is set as your startup disk? – Adnan May 16 '15 at 14:03
  • The OS in no longer installed on the Mini. It was destroyed when I splitted the fusion drive... Now the system by default boots into efi recovery partition which I think is hosted on the mechanical drive (so, disk utility cannot umount it). – MadHatter May 16 '15 at 14:05
  • To unmount the mechanical disk you need to start your mac mini using a USB stick with Mac OS installer. Here is how to make the bootable USB stick using DiskMaker X: liondiskmaker.com (for this you need a working mac) – Adnan May 16 '15 at 14:40

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