Changing a DVD-RW from Read only to Read-Write?

When you open the System Profiler does it say your Superdrive has DVD-RW support or just DVD-R support?

Here is the link which may help you:

http://docs.info.apple.com/article.html?path=DiskUtility/10.5/en/duh6.html

Note, with RW disks, you can write until full, and then erase and write again.

Ah error code Ox80020042. It is a common issue. I don't know what causes it but it has been reported a lot in the forum.

A friend informs me that the error code simply means the disk is not writable. So there is some reason why the disk you are using is losing its writability. Whether it is because the tracks burnt have lost their ability to be written to, or the laser used to burn isn't working properly, it is hard to say. Try different media and see if it makes a difference. If it doesn't I'd say the drive may need to be repaired or cleaned.

Yep - 0x80020042 is the error kDRMediaNotWritableErr, which simply means the device couldn't write to the disc.