My late 2009 iMac 27" needs the volume repairing but I have a problem. The drive will read and play audio c.d.'s but refuses to read Dvd's or Apple software disks - even physically marking them when I eject them. Is it worth trying to do a forced install by holding down key C on booting up or is this unlikely to work ? My other option is to get an external drive but do all standard ones read Mac disks ? Could I use target mode from another iMac with the Firewire cable - or even create a bootable USB to resolve this issue ? Any thoughts gratefully received on best way to proceed !