DAO, Disk at Once CD/DVD Image (.dao)
The DAO Misc file format
Files with the DAO extension are created using CD/DVD duplicator software. This software is used by devices and systems that allow the exact duplication of CDs or DVDs. They are even able to duplicate several discs at the same time. DAO files contain the exact duplicated information from either a CD or DVD and are used for backup or distributional purpose.
Technical details of DAO files
Files with the DAO extension are an exact copy of a disc, may it be a CD or DVD. This means that extra information, which would get lost if files are just copied from one CD or DVD to another, are maintained and present on the copy as well. Thus, an exact version of the former original disc can be created out of a DAO file. Thanks to the Disc-At-Once (DAO) mode of copying, the contents of a CD or DVD are copied in one go. That way, audio data (if present) can be written in the gaps between tracks on a disc. Contrary to that, the Track-At-Once mode transfers data track by track, where this kind of information is lost.