Of course when you get the question mark it means your unit can't find an operating system. If you've replaced the drive, even if you haven't, I would reformat the drive and use the option "write zeros". This will map out any bad blocks on the hard drive. A bad block will cause your system not to work sometimes when part of the system gets written to it. As a matter of course now, I zero out every hard drive I get, especially brand new ones. These hard drives are made as cheaply as possible and quality controls allows certain percentages of drives to be defective. So do your own quality control and you won't have to worry about this potentially devastating problem.