What is a Multisession Disc?

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A multisession disc is a disc to which data is added incrementally in more than one recording, allowing you to add, update, or delete files and directories. All the data on a multisession disc can be seen as if it were all recorded at the same time (if you linked the data between sessions).

On a multisession data disc, each track of data is recorded in a single session which is closed after the track is recorded. When the session is closed, a session lead-out is written which allows a disc drive to recognize the session and read data from it. Writing the session lead-out, and a lead-in preparing for a new session you'll write in future, takes up space on the disc: 22 megabytes for the first session, 13 megabytes for each subsequent session.

When you record the first session on a disc, the names and addresses of the files recorded are written in the file system for that session. When you add more files in a subsequent session, a complete new file system is written for the new session, but it can include references to files recorded in the previous session; this is known as linked multisession. The actual files already on disc do not have to be written again in the new session; only their addresses are included in the new session's file system. These addresses can be carried forward in additional sessions, so that all files recorded in previous sessions are shown as if they were part of the latest session. When linking data between sessions, you can virtually "overwrite" an older version of a file already recorded on disc by writing a newer version of the same file (with an identical filename and directory path) in a new session. The link to the previous version of the file is lost, but the file itself is still on disc, and you can get it back again if you need it.

If you do not want to add more data to a multisession disc in future, you can finalize the disc. After this it is not possible to write to the disc anymore. However, a finalized rewritable disc can still be erased.

Multisession is useful in making incremental backups, or distributing data which is periodically updated.

Note: Packet writing, another approach to adding data incrementally to a disc, is not the same as multisession. Packet writing is supported by Adaptec's DirectCD software included with your unit.

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