Can't unmount the disk during Mac OS X install My MacBook started to get really slow recently. Now it can’t start up properly anymore. All I see is the Apple logo and then it shuts down. I've tried rebooting multiple times, but I've seen the same result over and over again.  

So, I tried to reinstall my OS, but for some reason I get the error I cannot unmount my disk. And the partition seems to be locked, so I can't reinstall the OS over the internet.  

Do I have a damaged disk?
You cannot unmount the disk you're working on, obviously.
Try to boot from a different media type other then your HD (USB-drive, DVD,...) and then try again.

(Mountain) Lion allows you to create a bootable USB-drive or DVD. If you do so, you'll be able to unmount (and format) your internal Macintosh HD.