Hello, I use my iMac probably for several hours every day and have not had any problems with lag or any other issues. Today, I was using my computer while making dinner. I came back from dinner and noticed the computer was turned off. When I turned on, it crashed from the Apple logo. The DU first aide fails when running. This leads me to believe the drive may be failing - which is weird to me considering everything was working very well. Not to mention that my iMac is about 3 years old, where as the mac I'm using now is from 2009 and use well over 6 hours every day :/ Before buying a new drive and paying for the repairs, I want to try to perform a reinstall of the OS. When I try to perform an install of the OS I am told the hard drive is locked. When I try to unlock the hard drive in disk utility, the option to choose "Unlock" is greyed out and I cannot press it. Any suggestions? I won't have means to make a backup until Wednesday, so I don't want to do a formatting until then. If anyone has any advice, I would be grateful.