HI; hope someone can help with this as I am a bit stuck. I am running snow leopard on mac pro 2008 2x4 xeon 32G ram. Have installed mountain L. yosemite also. On mountain l. my cursor has stopped working; it just kind of jumps. I can get it to where i want it if i really concentrate moving it very very slowly. On yosemite it doesnt really work at all and i can't type my password even; if i type one letter it types about 6 and so on. I tried installing el capitan, but I am stuck at the place where it says "set up installation" as my mouse and keyboard don't respond at all. If i try restart and option key, it has stopped showing me the snow leopard option (this is the only one that works ok with the mouse and keyboard) and whichever hdd i take out it still seems to show me the "install OSX" page (el capitan) which I am stuck on. Snow leopard is on a separate hdd. This is really driving me nuts; if anyone can help I would really appreciate it.