I have an older 27" iMac that I primarily use as an iTunes library server for my Apple TV. I don't use it for much these days, but I noticed that it froze up today with some graphical corruption on the screen. When I rebooted it, I saw 5 vertical pink stripes over the Apple logo, then the screen went white, and the system rebooted. Wash, rince, repeat; same thing over and over. I tried to boot into the recovery disk, and it just stopped on the white screen. Left it unplugged for an hour, same thing. Is there anything else I can try with this system, or is it pretty much toast? If the latter, is there any way that I can securely wipe the hard drive to sell the remnants of it to Gazelle or somebody?