Should I create a Fusion Drive before or after downgrade (from Yosemite to Mavericks)? I bought a used Mac Mini late 2012 with installed Yosemite and two drives: SSD 128 GB and HDD 500 GB. I want to create a Fusion Drive and downgrade to Mavericks via bootable USB flash drive. What should I do in which order?
1) Boot into recovery HD and create a Fusion  Drive (erasing all data)
2) Install Mavericks on SSD part of Fuzion Drive?

But if I already have a Fusion Drive how do I choose explicitely that I want OS to be installed on SSD not HDD for performance boost?
Your first idea is the only way that it can be done.


Boot into recovery HD and create a Fusion Drive (erasing all data)
Install Mavericks on SSD part of Fusion Drive


A Fusion drive is for all intents and purposes a single drive, the OS sees only one Volume. The promise of a Fusion drive is that it will optimize the contents in such a way that you will receive the highest performance possible while having a large volume to store your data in.