1. A method for creating filesystems on a computer readable media, the method comprising the steps of: defining a filesystem structure for each filesystem, each filesystem structure comprising a hierarchy of filesystem objects, and each filesystem object having at least one call back function that identifies the source of data for the filesystem object; storing the filesystem structures in memory without storing the data from the identified sources as part of the stored filesystem structures in an image of each filesystem, the filesystem structures are less than a complete image of each filesystem; and writing the filesystems onto the computer readable media from the filesystem structures by reading the callback function for each object from the filesystem structures stored in said memory, retrieving the data for the object from the source indicated by the callback function, and recording the retrieved data on said media, wherein the callback function causes the data to be computed in real-time; and wherein computing the data in real-time includes a process selected from the group of: decompressing the data from a compressed archive and decrypting the data from an encrypted archive.