HT1284: Mac OS X v10.5, Mac OS X v10.6: When to use Time Machine or Backup to back up dataLearn about Mac OS X v10.5, Mac OS X v10.6: When to use Time Machine or Backup to back up data
Time Machine makes hourly backups automatically. As you don't have the external drive permanently connected, those backups aren't made, so press the Time Machine icon on the menu bar and select the option to make a backup now, and see if it's made correctly. If you want more information about your backups, open System Preferences > Time Machine.
If you want more information about Time Machine, visit the Pondini's website > http://pondini.org