Hello, all, I have an early 2009 Mac Mini and a couple of years ago I replaced the optical drive with a small SSD to use as the startup drive, leaving the original HD for media files, libraries, etc. They work great together, without quite being a homemade Fusion Drive. Tonight I was looking at the Startup Drive option under System Preferences and realised that when I upgraded to Yosemite last year, it only upgraded the SSD, leaving the HD still listed as running Mavericks. I don't perceive any problems in daily use, but I just wondered if it was preferable to have both running the same OS, and if so, how to do that. I ask here because I can't seem to find the right Google terms to get an answer. Thanks in advance for your help.