7

It just interest me how, no matter what computer I put it in, whether it be running Windows, Mac or Linux, I can't seem to format it (I know that they're made to be write-once only). I'm guessing it's a hardware thing. But even so, what stops a computer from ignoring the rules and formatting the disc anyway?

1
  • 1
    Same reason as why it can not write to a CD (hole and lands are fixed. For writing it needs to be flexible dye based. See superuser.com/questions/530139/… for a similar answer, just replace DVD with CD).
    – Hennes
    Mar 13, 2016 at 18:55

2 Answers 2

16

Put in fairly simple terms (and how I understand it, I could be slightly wrong about the actual manufactured materials, but I believe the process is as follows):

  • Pre-recorded discs have small holes in the surface that will prevent the reading laser from being reflected, giving you a reading of a 0 or a 1.
  • Recordable discs have a dye that can be burned through by a drives writing laser. The gaps in the dye now work the same way as a pre-recorded disc would, representing a 0 or a 1 based on whether it is reflected back or not. Once this dye has been burned through, it cannot physically be re-recorded (although I guess you could burn out the entire surface but not make anything useful).
  • Rewritable discs use a type of metal surface instead of a dye, that can be changed by the write laser, depending on the power of the laser used on it. This makes the metal layer reflect differently where the laser has been, and can be "reset" by a different powered laser.

As such, a writable disc is permanently "set" in the way it is by a write laser, with no way to reset the damage it does to the dye in order to write the data.

Further reading: All about CD-R and CD-RW (albeit related to CD-R/RW technology)

2
  • 8
    Note that it is possible to further extend a DVD if you record it with the UDF filesystem (and don't "finalise" or "master" it in the traditional way). But this never overwrites new data; it merely appends new data to the end. You can also "delete" things but you never get the space back and the "deleted" data might remain - they're just marked as deleted in the newly appended section..
    – Bob
    Mar 13, 2016 at 13:05
  • The question was about why the software can't still attempt to burn to a disk that's already been burnt to (pre-recorded or otherwise), not about why burning to a write-once disk more than once is ineffective. Mar 13, 2016 at 18:27
-1

Some disks contain special information indicating that they are read-only, or that they are music CDs, or other things like that. This information is contained in a small circular barcode near the central hole in the disk. If the drive thus determines that the disk is read-only (such as a pre-recorded disk) then it will physically refuse to write to the disk.

In other cases, the hardware or even the filesystem drivers prohibit writing to the disk if they determine that it is read-only (such as if it is write-once media). Try to mount pretty much any optical disk read-write under Linux and you'll get a message saying "the device is write-protected" (or whatever the exact wording is). This is the equivalent of those movable "tabs" that were on floppy disks, or the "write-protect" notches on some SD cards (and flash drives?) - basically, there's nothing physical or necessarily even hardware-related to prevent writing to the disk, but the driver won't permit writing because the driver knows that the user is never going to want that (because they've explicitly write-protected their disk, or attempting to write to the disk would destroy the data, or whatever).

There's another issue at play here as well though, and that is that burning to optical media is quite different to mounting it read-write. When you burn an optical disk under Linux, you don't mount it read-write and then copy whatever files you want onto it. You can't even run mkfs on an optical disk drive. Burning to the disk puts the drive in a different "mode", and entering and operating in that mode requires specific software. That's where programs such as Linux's wodim/cdrecord come in - these programs can put the device into "disk burning mode" and then send the appropriate commands to burn the data to the disk. Again, as with the device driver (and the hardware), these programs may perform a "sanity check" to make sure that the disk being written to is blank or re-writable to avoid damaging the disk.

Not the answer you're looking for? Browse other questions tagged or ask your own question.