Multisession Support
ISO 9660 is by design a read-only, pre-mastered file system. This means that all the data has to be written in one go to the medium. Once written, there is no provision for altering the stored content. Therefore ISO 9660 is not suitable to be used on random-writable media, such as hard disks.
The Multisession extension to ISO 9660 makes use of this feature, by defining a rule for operating systems as to how to read an ISO 9660 volume from a CD-R. Instead of looking for the volume descriptor at offset 32768 (block number 16 on a CD) from the start of the disc, it starts reading from the 16th block in the first track of the latest session. Block numbers form a contiguous sequence starting at the first session, and continuing over added sessions and their gaps.
Hence, if a CD mastering program wants to add a single file to a CD-R that has an ISO 9660 volume, it has to append a session containing at least an updated copy of the entire directory tree, plus the new file. The duplicated directory entries can still reference the data files in the previous session(s).
In a similar way, file data can be updated or even removed. Removal is, however, only virtual: the removed content does not appear any more in the directory shown to the user, but it is still physically present on the disc. It can therefore be recovered, and it takes up space (such that the CD will become full even though appearing to still have unused space).
It should be noted that it was originally intended that multisession support only applied to Mode 2 Form 1 formatted discs. However, the specification was somewhat ambiguous on the point and some CD writing software supported multisession writing to Mode 1 format discs. Unfortunately, only some of the early disc drives themselves supported multisession Mode 1 discs, although they were quite happy to look for the volume descriptor for the first session, they would not look for the volume descriptor for any subsequent session. Thus such drives would only allow the reading of data recorded in the first session but not any subsequent session of Mode 1 discs.
Some older CD writing software, such as Nero, would not import previous session data from an inserted disc, and thus could only write a subsequent session to a disc, on the computer that had written all the previous sessions and then only if the previous session data was saved before the writing software was closed down.
Read more about this topic: ISO 9660
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