My iMac 2008 suddenly doesn't load past the white screen. It has been running smoothly on OS X Yosemite since June, just today gets stuck on white screen - sometimes the flashing question mark appears but not every time. It doesn't go past it even when I hold the Option Key or Control + R for several minutes. Last Install DVD I own is Snow Leopard but it gets ejected quickly. I have a Time Machine on an external drive that has not been updated for a couple of months - I worry I might loose all the new data unsaved if I try this last resort.