I upgraded from Snow Leopard to Mavericks to Yosemite on 20" iMac. The device is functioning unbearably slow. I have the snow leopard DVD that came with the computer. But the optical drive is not working on it anymore. The optical drive is not available as the product now is obsolete. I attempted to install from recovery partition, it did not work as there seems no recovery options when I boot to that window using Ctrl+R. Living in an area where I cannot arrange for another apple computer or laptop so I could install OS via another apple drive. The only option left is to copy the contents of DVD to a USB and install from it. However, not sure how I can achieve that. Need help!.