So I finally got around to downloading yosemite and I thought it would be an easy download, but when I restarted my computer it continually kept saying the file was crashing or it was corrupt. After looking for a solution I decided to delete my PRAM, which seemed like it was helping, until I arrived at this impasse. It now won't let me open the download the new OS. At first it just says the mac is locked, but after going back and forwardagain it says it cannot be used on this mac. I'm not sure what my next move should be. Since I was expecting this to be a quick and painless download, I didn't back up my files (stupid in retrospect). I just don't want to go through the process of deleting my old OS and doing a clean download, because I don't want to lose the data I have, that hasn't been backed up. I'm currently trying to see if my mac will open in safe mode, but as of now no luck. If you have a solution or any ideas I would appreciate it.