I have a mac mini server with 2 partitions. The first houses Mac OSX Yosemite, the second Mac OSX 10.6.8. I use the late only to play Mahjong from double, and keep changing start up disks frequently. Well, I upgrade to Yosemite and after that went back to the 10.6.8. To my surprise, I can't go back anymore because OS Yosemite doesn't appear as start up disk in that startup disk preference in System preferences. Trying to boot while pressing option key doesn't work also. What could I do to fix this problem?