How to switch off write protection?

Trying to save Autocad files to a previously saved drawing file on CD RW disc (UDF formatted) produces the message 'Drawing file is write protected'.In other words i can save a drawing in the first place , or create it, paste it , copy it, whatever I like, but once created on a CD RW disc i can't save to it or amend it again.I have to copy the drawing to another location on the hard drive, then close the drawing, and using windows explorer copy the file back to overwrite the original drawing.In drawing properties the box saying 'read only' is unchecked so this isn't the problem.Nor is there a problem with the same file on a floppy or any hard drive... so what is it about CD -rewriters that won't play ball?I can't see that it is an autocad issue, after all its a file like any other...but i may be wrong.
Be grateful for any input!