Apple's wonderful Disk Copy utility gives you the ability to create a disk image - a file which pretends to be a mounted disk - which the operating system treats as a real disk. This is a short tutorial that explains how.
You'll find the Disk Copy utility in the Utilities Folder, which is inside the Applications (Mac OS 9) folder on your OS 9 computer. There is also an OS X version included with newer machines that does exactly the same thing.
The process is actually very simple. If the CD-ROM consists of simple files and folders, with no encryption or hidden files, simply insert the CD-ROM in your drive and wait for it to mount. Then start up Disk Copy and choose Create Image from Disk under the Image menu.
Disk Copy creates a disk image file that contains the actual data. This file appears on your desktop - or wherever you specify to save it - as a file ending with ".img". It then proceeds to attach to this file information which is exactly the same as found in the hidden directory structure of any mounted drive on a Mac. This includes the desktop information file, for example, that users cannot normally see or directly access.
After zeroing out the new "disk," the OS is told to mount the file as if it is an external drive, such as another CD-ROM or a FireWire drive or whatever. It then appears on your desktop as a blank disk. When you choose the "Create New Image" command, and the disk is ready to receive data, just drag the contents of the CD-ROM onto the mounted image, and, in many cases, you're done.
Since no one has an infinitely large hard drive, you should do a "Get Info" on the source disk to see how much data there is to copy. When Disk Copy asks you to set the size of the image file, your image file should be at least this large.
On simple CD-ROMs, this works great. You can remove the CD-ROM and return it to your protected archive (read: Kmart CD pouch). Then, when the disk is needed on the computer, you just double-click on the .img file and the copy "mounts." No CD - no problem.
You would think that many manufacturers would take steps to prevent such easy duplication, and you'd be right. Many software disks, at least for the Mac, contain a few hidden files that the Mac OS doesn't display and cannot be manipulated with a mouse. These files do not show up in a directory listing, and the don't add in to the total file size in a Get Info. Thus, when you use a "Create Disk Image," they aren't copied because you didn't allow enough room for them. When attempting to use the mounted image, some file that is crucial to the program (but invisible to you) will inevitably be missing.
The solution is pretty easy. Instead of using the "Create Disk Image" command, use the "Create Image From Disk" command. This will copy the invisible files as well. You still need to estimate the image file size, and if you are trying to save hard drive space that may take some trial and error. But if you have an 80 GB hard drive, it will hold many CD-ROMs without any difficulty, so you could specify a size slightly larger than a CD-ROM's capacity and get every file possible.
On older Macs, you might have to play around with the size a bit for maximum efficiency. Machines with drives smaller than 1 GB will probably have difficulty preparing more than 1 or 2 CD-ROMs as disk images. You also get a compression option when saving the image, so this would be a good time to use it.
If you place the image or an alias of it in your Startup Items folder (inside the System Folder), it will automatically mount with verification when you boot your machine. If you want to get rid of the verification step - which is really unnecessary since you have the original CD-ROM anyway - then you can write a short AppleScript to mount the images without verification, a command that is understood by Disk Copy when in a script.
I have used this technique on as many as 20 CD-ROMs (all of which I own) simultaneously, and all the the disks mounted just fine on the desktop.
Of course, you shouldn't exchange the image files unless both parties involved own the software. Also, this technique doesn't work on audio disks; for that you have iTunes.