Best way to back up old CD-ROMS?
Personally I'd forget the dmg format, buy a decent sized usb drive and retrieve as much as you can by just dragging and dropping from any CDs you can mount. Time enough when you have retrieved all your data to wonder about how to compress it. Even if a CD is full it can only store 700mb so it would take a load of CDs to fill a terabyte drive. Then, when you have all your data saved, make another copy and store that offsite somewhere as a backup.