Open a .toast CD image
A .toast file is a CD image made by Toast, the Macintosh burning software that everyone with a CD writer owned. Drop one in and it mounts as a CD on an emulated Macintosh, which is generally the only thing that will read it properly — Toast images of Mac CDs use HFS, which Windows cannot mount at all.
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What is actually in a .toast file
Usually, a plain track image with no wrapper — which is why renaming a .toast to .iso sometimes just works. Sometimes not: Toast could write multi-session discs, hybrid discs with both an HFS and an ISO 9660 filesystem, and images with a small header, and those need something that understands the format.
The hybrid case is the interesting one. A great many Mac CD-ROMs from the 1990s were hybrids so that one disc would work on both platforms, with the Mac files in an HFS volume and the PC files in ISO 9660, often sharing the same underlying data. Mount such a disc on Windows and you see the PC half and conclude the Mac half is missing. It is not; you are looking at the wrong filesystem.
Why the CD-ROM era matters here
Between roughly 1993 and 2000 the CD-ROM was how Macintosh software arrived: 650 megabytes when a hard disk was 250, which meant full-motion video, recorded speech, and enormous art. Myst, the Living Books, the Encarta-era reference discs, the shareware compilations that carried thousands of programs each.
That generation is the hardest to preserve and the hardest to run. The discs rot, the drives are gone, and the software inside frequently will not run on anything newer than Mac OS 9 because it depends on QuickTime 2 and a 256-colour screen. An emulated Macintosh with the disc mounted is often the only remaining way to look at them.
If your disc is on a real CD
Make an image of it before the disc degrades. On a modern Mac, hdiutil will do it from the Terminal:
hdiutil create -srcdevice /dev/disk4 -format UDTO mydiscwhich writes mydisc.cdr — a raw track image you can rename to .iso and drop in here. On Linux, dd if=/dev/sr0 of=mydisc.iso bs=2048 does the same job. Both preserve the HFS volume, which is what matters.
Questions people ask
What is a .toast file?
A CD image created by Toast, the standard Macintosh CD burning application. Most are plain raw track images; some carry Toast-specific structure.
Can I just rename .toast to .iso?
Often yes, for a single-session image. It will fail for multi-session images and for anything with a Toast header.
Why can't Windows read my Mac CD image?
Because it is an HFS volume and Windows has no HFS driver. The data is there; Windows simply cannot see that filesystem.
What is a hybrid CD?
A disc with two filesystems — HFS for the Mac and ISO 9660 for the PC — so one disc worked on both. Mounting it on the wrong platform shows you half the disc.
How do I make an image from a real disc?
hdiutil on macOS or dd on Linux. Both are shown above.
Will CD audio tracks work?
No. Only the data track is mounted; games that played music from audio tracks will be silent.