I should probably add to gh0sth@cker's comments, that we're talking about BACKUPs here, for personal use.
If you want to avoid having your original game cd getting eaten by the dog or a passing dinosaur
, you'll need to make a backup and play using that backup while the original is somewhere in a dog-and-dinosaur-proof safe. Most game compagnies these days use all kinds of copy protections to prevent us from doing this, hence the existence of this forum.
There are two kinds of cracks : no-cd patches, which allow you to play the game without any cd at all in your cdrom drive, and fixed executables, which allow you to use a backup cd, which must be present in the cdrom drive for the game to work.
There's a third alternative which doesn't need cracks and that's to make an exact copy of the original cd. Mind that this doesn't always works, especially with safedisk 2.51 and higher.