The Table of Contents (TOC) on a CD-R is the repository of all the information required for your OS to understand the pattern of bytes on the disc. Essentially, it contains everything needed for your system to figure out what information is where and in what format, so that your request to receive information can be satisfied.
For example, on an audio disc the data are not stored in files, but in a continuous stream. The TOC informs the OS (or the player) that a specific block is the beginning of a track. When you play a track, a command is given to go to that block and to begin to retrieve the bit stream which follows. Note that the track itself - in the audio sense - need not begin where the TOC says it does, but that is irrelevant to the player. There are also subcodes which can be used to interpret how to backspace and so on - but they are beyond the present scope. (And if you want to read that as meaning that I don't understand them, I couldn't argue.)
When you go to write a Disc At Once (DAO), all the information needed for the job is available when you start. Mastering software assembles that information and decides what is to be in the TOC. When everything is ready, the laser is positioned and turned on and writing begins. The first thing written is the TOC which says: this is a closed disc of a single session and here is the information on what you will find where among the following bytes. The laser then proceeds to write those bytes in sequence and in accordance with the information saved to the TOC. When the last byte is written, the laser turns off and you have a good disc. If that burn is interrupted after the TOC is written but before the end, everything is accessible up to the failure. An audio disc which fails when track 5 is being written will be usable for tracks 1-4 and maybe for the written part of 5 - but what happens when the track runs out depends on the player.
When you write Track At Once (TAO), the procedure is quite different. The writer begins by inscribing a special area on the blank called the Program Memory Area with information needed to close the session later. Then it records track information and writes the track itself. When it's time to close the session, the writing laser turns off, the mechanism returns to the PMA and the information inscribed there is read back to close things up, record how many tracks there are in all and, in general, to complete the TOC. So if the burn fails before the PMA is read (or if the PMA cannot be read at all), the TOC is not complete and a reader cannot make sense out of it. However, not all is lost! Although the reader needs a complete TOC to do its job, a writer is prepared to make do with less and, in fact, has what it needs in the part of the TOC which is complete to retrieve the valid data.
Suppose that you have written your disc - and the software reports errors. If the error has to do with reading, then it is almost certainly the inability to read the PMA when it came time to close the session. The disc is trash, but you can retrieve the information from it in a writer. That information may not include what was on the last track, so if it was a data session you're in trouble. But if it was an audio session, you should be able to recover all the tracks but the last. Note that the particular error message you get is critical here. If it says buffer underrun, the data are not there so they can't be read and (in TAO) the TOC was not written, so you couldn't find them if they were. If it reports a communication error, it might well have occurred when seeking for the PMA; in that case, your writer may actually be able to read everything on a TAO disc. Finally, if a communication error came at the end of a DAO burn, it may mean only that the runout track was incomplete. Since most modern readers need very little runout, the disc may work perfectly for you although it is 'too long'.
E-mail me at cdrecording@mrichter.com
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