I picked up a 2009 17" mac pro that had os yosimite on it and when i attempted to upgrade to el capitan i booted to a prohibitory symbol, circle with line thru it..i do not have a backup of yosimite saved. Ive tried to restore the computer , control/r reinstall os, but it keeps trying to install el capitan which appearantly is not compatible with this mac, hence the prohibitory symbol..any advice on how to clear the pending update and restore to a compatible os ? The cd drive does not work but i can purchase a external if needed, and i do have a snow leopard disk to boot/install this os, as its supposed to be compatible, then upgrade back to yosimite.will this work? Any other suggestions?