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First, did you do it in Recovery, pressing Command and R keys when your computer started? If not, start in Recovery HD and repair the disk.
To make a backup, use Time Machine or clone the disk using Carbon Copy Cloner. In this case, it's better to make it with Time Machine.
To erase the disk, do the same you did to repair the drive in Recovery, but erasing the drive. Then, close Disk Utility and reinstall Lion. Setup Assistant will ask you to restore the backup