Actually, in the UK, you’re not if they’re in a different format (same goes for ripping music CDs to MP3) – see http://www.theregister.co.uk/2015/07/21/govt_messed_up_home_taping_exception_so_badly_it_needs_to_start_again/
But that’s by the by :)
In one specific case, I want to play Puggsy. Mega Drive version, easy to find. Sega CD version, easy to find. Mega CD version? I’ve found loads of images but they’re all corrupt or have no sound. But I do have the game sat on my shelf!
I have tried making an image with Disk Utility, then converted it to ISO with this command:
hdiutil convert image.cdr -format UDTO -o image.iso
but I get no CD audio. It’s not just RetroPie that doesn’t play the audio either, OpenEmu does the same. I’m guessing it’s because I need a cue file?