Vista - Vista burning feature not recognizing its own disk

I only use Vista built-in burning feature to backup important files. I’m not
interested in burning movies, music, make bootable disks, etc. I have only
one DVD that I use all the time until it is full. The problem is when
occasionally I want to add a few more files to the DVD, Vista generates this
request:

"Windows can’t write to the disk you inserted.
Please insert a writable disk into drive D:"

What is going on? This is exactly the same disk that Vista wrote to an hour
or a week ago. The only way to fix it is to copy everything to the hard
drive, format the disk and burn everything back to the DVD!!! This issue just
seems to be happening randomly. Any suggestions?