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.
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 http://superuser.com/questions/530139/what-prevents-cd-r-from-being-rewritten/530144 for a similar answer, just replace DVD with CD).
– Hennes – 2016-03-13T18:55:21.230