I found a simple solution to prompt my old Toshiba DR340KU DVD recorder to finalize a failed recorded DVD.

If the message: "Recording error. Cannot record on this disk. E03"

Step 1. While the DVD is still in the DVD recorder try finalizing the DVD disc using the Finalize function from the DVD menu 2 more times and if the same error message appears next try step 2.

Step 2. Eject the DVD and copy the DVD to another blank DVD using Nero Burning ROM or any other DVD copying software on your computer.
Step 3. Once the copying process has finished eject the copied DVD from your computers DVD drive and insert the copied DVD into your Toshiba DR340KU DVD recorder and select Finalize from the DVD menu.
Step 4. Eject the DVD disc from the DVD recorder and test the disc for playback on your PC. if the DVD disc plays back the disc has been successfully finalized for playback on your and any other DVD player.