Re: Can't boot from live media
I can't boot ubuntu on my iMac (21.5, mid-2010). Like you, all media is rejected.
My internal DVD has failed. An external DVD won't boot from DVD.
The only (partial) success I've had with linux on iMac is to put fedora (20) on USB. Runs well but has radeon video driver issues.
It will boot OK and install - CAREFUL - it might also set itself to boot in preference to OSX.
To get OSX back you press Alt during boot and select Recovery image, then set Startup disk from Apple menu.
I think only fedora has a UEFI-boot-compatible image. (Maybe Archboot will do it too?).
I think this is probably a fail for linux packagers.
Windows is going exclusively UEFI-boot so recent PCs are set to UEFI mode in BIOS.
Linux live DVDs should be set to UEFI boot, or at least have a UEFI boot version.
I note that clonezilla is now UEFI (and Secure boot) compatible, so it can be done.
Windows is cheap, but linux is free