Best way to back up old CD-ROMS? I'm trying to backup all of my old backup CDs from over 10 years ago to simple DMG files on my harddrive - since CD's die (already have one that won't read) and they take up a ton of physical space!

I've picked DMG files since they're 1) simple/easy and 2) compressed. Certainly open to other suggestions however.

After creating a few of the images, I've noticed that not all of the content from one of the CDs is in the image that Disk Utility is producing - only one of the folders is present when there should be 4. I can't figure it out, but my best guess is that it was a multisession/track CD. But the CD doesn't appear as unique in DiskUtility or in the Finder.

Any ideas what could be the cause? Or is there a better way to do what I'm trying to do?
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.