Unlike other systems, Mac OS doesn't treat CD images as read-only. A HFS or HFS+ volume is treated the same whether it's on a floppy, hard disk, CD, zip etc. Unless the media is write-protected, it's fair game for writing to. And Mac OS (especially OS X) loves writing things to volumes.
From a software preservation point-of-view this is a nightmare and it's sadly all too common to find valuable CDs imaged incorrectly. Yes it all might sound a bit anal, but really if it's not hard to do it right, why do it wrong?
So here's a couple of quick pointers on how to image a CD correctly and not mess it up afterwards. The key to this last part is to ensure than the image file is read-only before attempting to mount it. This has all been covered before in different parts of the forum but it may be buried too deep for some to find.
To image a CD in OS X you just need the built-in Disk Utility. This will also handle Mac/PC "hybrid" discs (but not CDs with audio).
Pop in your CD and launch Disk Utility. Click on your CD/DVD drive (not the volume) in the drive list (Fig.1) and then click on the "New Image" button on the toolbar. Set the output type to "DVD/CD Master" with no encryption and save it out (Fig.2).
Once the image is created, right-click on the image (or click and go to the File menu in the Finder), choose "Get Info" and tick the "Locked" option (Fig.3). Now the image is read-only and won't be modified if mounted (Fig.4).
A DVD/CD Master .cdr file is just a plain flat disc image, so you can change the file extension to .iso if you like.
Toast is the weapon of choice for imaging CDs in the classic Mac OS. You will need at least version 5 if you wish to image Mac/PC hybrid discs (but not CDs with audio).
Once completed, right-click on the toast image (or click and go to the File menu in the Finder) and choose "Get Info" and then "General Information" from the options (Fig.8 ). In the Info dialog box, tick the "Locked" option (Fig.9). Now the image can be safely mounted without being modified by Mac OS (Fig.10). If the disc was a hybrid, Toast will mount both "sessions" on the image as seen here (Fig.11).
Windows / Linux etc
Pretty much any CD tool on these platforms will be able to successfully image a Mac CD. Just because Windows can't mount a Mac volume (without third-party software) doesn't mean it can't be copied. ImgBurn, Alcohol, Nero etc can all make a proper ISO image of a Mac CD.
On Linux the situation should be the same (you could even do a device copy using dd from the terminal).
As a precaution it's good to make the iso image read-only before compressing it to share. In some cases it will remain read-only when extracted on a Mac and thus will not be modified if mounted. Though it's good practice to always check if an image is locked before mounting.
To get a proper 1:1 copy of a CD with audio tracks you need to use a tool that can create multisession images. In OS X 10.4+ you can use SimplyBurns or FireStarter FX. For older versions of OS X there's MissingMediaBurner. These tools will create a bin/toc image (the bin file contains the raw data and the toc file contains the necessary track info). It's the linux equivalent to the more widespread bin/cue format that's commonly used in Windows. There are no tools to create these in the classic Mac OS.
For Windows you can use a tool such as Alcohol for bin/cue or burnatonce for bin/toc. In Linux any tool that's based on the cdrdao engine will output a valid bin/toc file that can be burned on OS X by SimplyBurns, FireStarter FX or MissingMediaBurner. For more info check out the discussion here.