Hybrid disc

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A hybrid disc is a disc that is simultaneously both an ISO 9660 disc and an Apple disc. This allow the disc to function on a variety of computer operating systems such as Microsoft Windows, Linux and Mac OS.

A hybrid disc has an ISO 9660 primary volume descriptor, which makes it a valid ISO 9660 disc, and an Apple partition. It may also have an Apple partition map, although this is not necessary.

The ISO 9660 portion of the disc can co-exist with an Apple partition because the header areas which define the contents of the disc are located in different places. The ISO 9660 primary volume descriptor begins 32,768 bytes (32KB) into the disc. If present, an Apple partition map begins 512 bytes into the disc; if there is no partition map, the header for an Apple HFS partition (known as a Master Directory Block, or MDB) begins 1,024 bytes into the disc.

Hybrid discs are often used for distributing computer software. For example, the ISO 9660 section of the disc can hold software for the Microsoft Windows platform, while the HFS part of the disc holds Apple software.

In a true (or shared) hybrid HFS filesystem, files common to both the ISO 9660 and HFS partitions are stored only once, with the ISO 9660 partition pointing to file content in the HFS area (or vice versa). In effect, the ISO 9660 and HFS areas of the disc overlap.

In non-shared hybrid discs, the ISO 9660 and HFS areas are completely separate from each other, and the content of any files common to both partitions is duplicated.