How to repair OS X Snow Leopard from USB Stick My operating system (Snow Leopard) was running quite good but I used some tools from internet to free some HDD space. Then some days later I figured out that Safary is not able to start anymore.

Now I'd like to repair the current installation or re-install the OS X.

But how can I do that?

I managed it to create a bootable USB drive from the installation image. When I boot from the USB drive, the installation asks me which drive is to be used. And when I select my laptop drive the installation program shows the message that this drive cannot be selected.

What should I do in order to force the installation to overwrite the current instance of OS X?
You must use disk utility to erase the hard drive -- the settings when you erase should default to macOS Extended Journaled (non-case sensitive) on GPT. Then close Disk Utility and open the installer (while in recovery of course).

Note: all data stored on the HDD will be erased. A full backup is strongly recommended before proceeding with the procedure listed above.