Erase Mountain Lion installer from USB drive

Hi everyone !

When Mountain Lion came out, I downloaded it and copied the image to a SD CARD so I could install it on my macbook air - without having to re-download it.

Now that ML has been installed on both machines, I want to erase the content of the SD CARD and use it on my camera. But when I go to the disk utility, the ERASE button is greyed out...

It must be a permission issue, but by popping the info menu, I can't even change the permissions...

Anyone has an idea?

thanks.

Chris.

(enclosed 2 screenshots - sorry it's in french.)