Mavericks won't install because Macintosh HD "cannot be used to start up your computer"
MacBook Air (11-inch Mid 2011), OS X Mountain Lion (10.8.5)
Thanks for the quick response. I had already found this while searching for a solution, and it unfortunately didn't solve the problem. I'm in the process of backing up right now, I wanted to avoid reformatting and reinstalling, but it's looking like that might be my only option.
Mavericks won't install because Macintosh HD "cannot be used to start up your computer"