Destination Backup folder not connected

I'm sitting here away from the desk working over wi-fi and notice the Espionage message "backups successful". Then I realized the folder where the backups are sent is on an external hard drive which is not connected to the laptop via USB. This sequence has happened many times. Suddenly I had the thought "how could the backups be successful if the destination folder for the backups is not even connected to the laptop?"
Is the backup going somewhere within the application, which resides on the laptop, and then transferred later to the external hard drive when available? Then I could see the "backups successful" message being correct. Otherwise, backups should not be completing and a message indicating "destination folder not available, backups aborted" should be displayed. Or some message indicating backups could not complete.
See where I'm going with this? What is happening with the backups when the hard drive, containing the backup destination folder, is not available?
Environment Info:
SyncServices is unlocked at login. (this results in a backup being done every 3 hrs, see below).
Preferences/Backup -- When folder locked and every 3 hrs when folder is unlocked.
All folders on the unavailable external drive(s) are locked before disconnecting.
All other folders on the internal hard drive are locked during the above described event.
Is the backup going somewhere within the application, which resides on the laptop, and then transferred later to the external hard drive when available? Then I could see the "backups successful" message being correct. Otherwise, backups should not be completing and a message indicating "destination folder not available, backups aborted" should be displayed. Or some message indicating backups could not complete.
See where I'm going with this? What is happening with the backups when the hard drive, containing the backup destination folder, is not available?
Environment Info:
SyncServices is unlocked at login. (this results in a backup being done every 3 hrs, see below).
Preferences/Backup -- When folder locked and every 3 hrs when folder is unlocked.
All folders on the unavailable external drive(s) are locked before disconnecting.
All other folders on the internal hard drive are locked during the above described event.