Trying to erase disk but "can't unmount"

Getting my macbook ready to sell and followed the directions here: http://support.apple.com/kb/HT5943
But when I try and erase the startup disk I get a failure message saying "can't unmount"
Will a simple reinstall of Mt Lion erase everything?  If not, please help...

exact wording in dialog box is:
Disk Erase failed
Disk Erase failed with the error:
Couldn't unmount disk.

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