Snow Leopard Won't Show Yosemite Hd


I installed Yosemite on an internal hard drive upgraded from Mavericks and I have another internal HD with Snow Leopard because I have some old app's, When I booted back into Snow Leopard then tried to boot back into Yosemite the Startup area in system pref's won't show Yosemite hard drive. I also have a third internal HD with Mavericks on it and if I boot into it then I can see Yosemite HD in the possible HD's to boot into. Help, do I need to now completel do away with Snow Leopard?
The only way to boot from within Snow Leopard to a HD with Yosemite is to completely shut down and restart with Option Key then you will get a list of all hard drives to boot into.