I have a 240GB SSD and 640GB HDD in my Mac Pro. Would like to know what is the best setup to run with SSD+HDD. I installed OS X Yosemite on SSD. I am planning to install a second OS, Snow Leopard, on the HDD. But then I would treat that extra bootable HDD like additional storage. Is that a good setup? Or I should just format the second HDD as HFS+ or FAT32+ drive and use it as extra storage? The reason I would like to install Snow Leopard is as a spare for any app that was stable in Snow Leopard before but turns out to be unstable in subsequent iteration. And also just in case I would need to re-format my SSD, I still have another bootable one while setup-ing my main one.