I had made a Yosemite boot USB drive before installing 10.10 just in case performance issues arose, and I'm glad I did because sure enough - performance became unacceptably slow after "upgrading". So I've double backed up all my data and am now about to do a clean install of Yosemite. Not planning to restore configuration-related data such as Applications or Library data. What I don't understand is, why are there two additional drives showing up in Disk Utility? The drive with Macintosh HD is of course my internal drive. And then there's my Yosemite installer. I don't recognize "disk2" or "disk3". Are they partitions? If so, are they partitions of my internal hard drive or the installer drive or what? This is not a fusion drive, just a regular HDD. Still researching... Thanks in advance.