I created a bootable flash drive with Mountain Lion on it. My computer has Leopard 10.5.8 installed, and after reading online I thought I could still boot form a Mountain Lion USB drive and erase the hard disk and do a clean install. I tried booting it on another 10.5.8 computer and it worked, just not on mine. I have a Core 2 duo processor and 2 gb of ram, when I click on the USB flash drive from the boot screen it just gives me a circle with a slash through it on a gray screen. Any idea what causes this?