At first, backup your data (if not already done), the issue as root in a terminal
Code:
dd if=/dev/zero of=/dev/sdX bs=512 count=1
Warning, this will erase the bootloader and the partition-table of the disk! Do not do launch that command if you don't know what that means!
Of course you have to replace sdX with your actual device. After that just reboot and start the install, it will see your disk as empty.
