Locate your MobileSync folder and the back-up folder should be there.
Mac: Macintosh HD> User> (User name)> Library> Application Support> Mobile Sync
Windows: ? (though it should be a similar path, just locate the MobileSync Folder).
In the event that your computer does crash, its actually very simple to create a new back-up of your device without syncing it, so it might be a little unnecessary to save your back-up to an external drive.