Reference  should include the p parameter in the tar extract command.
tar xvzf backup_4.31.tar.gz
tar xvzpf backup_4.31.tar.gz
The reasons that I recommend using an external hard drive include the following:
- People don't want to split files to put them on DVD and then reconstruct them to restore.
- The tar method onto a storage medium that is large enough to hold the entire archive file makes it easier to restore to bare metal.