In that case you can disconnect the pst-file from Outlook, close Outlook and
backup the pst-file directly to the DVD.
You can disconnect the pst-file and lookup its path via;
File-> Data File Management...
The button is called "Remove" but you are only removing it from the mail
profile and won't be deleting it from the hard disk. Before pressing the
Remove button, you can press the Open Folder... button to directly go to the
location of the pst-file.
When you want to view the contents again, copy the pst-file back to the hard
disk and remove its "Read only" file attribute.
You can reconnect to it in Outlook via;
File-> Open-> Outlook Data File...
For further details see this guide;
http://www.howto-outlook.com/howto/backupandrestore.htm