Why can't we write over a CD/DVD after it's been burned/finalized?
I was wondering why CD/DVD burners can't just overwrite a CD or DVD after it's been finalized and set to read-only mode. I understand that read-only mode prevents further writes, but can't a burner just ignore that and burn over the original data anyway?
I know you wouldn't really be able to store any new data on the disc because it would most likely corrupt everything, but for data-destruction purposes, what's preventing this?
Is there just no software made for this purpose? Does the firmware on CD/DVD burners boards prevent this? Or has nobody had enough interest because discs are easy to destroy anyway?
I understand that read-only mode prevents further writes, but can't a burner just ignore that and burn over the original data anyway?
Yes, if you modify the firmware on the burner then you could do that.
That means either having access to the firmware's source in the first place (read: support from the manufacturer) or some tricky manipulation.
I know you wouldn't really be able to store any new data on the disc because it would most likely corrupt everything,
Correct. Once CD-R or DVD-R (not -RW) is written to, part of the disc is physically changed. There is no way to revert this.
For more details please see this post: What prevents CD-R from being rewritten?
but for data-destruction purposes, what's preventing this?
Nothing but the safety checks in the firmware.
This actually exists and is called smart erase. It is Lite-On proprietary and I have never seen anyone use this.
With SmartErase users can erase the data on DVD±R (DL) and CD-R discs that contain confidential data and which should not be retrievable in any way. The feature works by overwriting the original data on the disc with random meaningless characters so that the original data can no longer be read by any drive.
Source
If your intent is to destructively erase data, then yes this is possible. But there are easier and faster ways. shredding, burning, melting, solvents. There simply has not been the demand to allow it, and some demand to prevent it. Optical drives are heading the way of the floppy, so like the optical serial mouse, this may never be commercial reality. Lightscribe was cool.
Remember, the code written is not really binary, there are limits to the run lengths of the mark and space ratios. You cannot "touch up" this code.
In the past I used dd myself to duplicate (to the sector) certain unfinalised DVD-R's burned by particular video cameras, when nothing else would work.
It would probably not be overly difficult to have firmware overwrite a portion of the disk so that it could no longer be read via ordinary means (if one wanted to destroy an entire disk, physical destruction is much more practical than overwriting). It would be difficult, however, to ensure that the data could not be recovered via microscopic examination or other similar techniques. While there might be some limited value in a level of erasure which would require some work to undo, it doesn't really fit well with either of the two main purposes of erasure: (1) make space available for other purposes, or (2) make information genuinely unrecoverable.
I could imagine some potential usefulness for a specialized CD-ROM drive which was designed to be capable of destroying information on a disk in very targeted fashion in cases where it was necessary simultaneously to ensure that some information on a disk was truly destroyed but still have the remainder of the disk remain usable as physical evidence (if a disk is copied and destroyed, it may be hard to prove that the contents of the copy match what was contained on the original). Such destruction, however, would require a burning laser significantly more powerful than would normally be required to store information onto a disk, as well as a means of examining the disk in much more detail than a typical drive would be able to manage.