The mirror mode is actually a special case, where the mirror_list.cfg configuration file is used - which content usually is:

all;/

Note that the system will also honor the exclude_list ! even in mirror mode - this to prevent backuping stuff we don't require.
The mirror mode will make a backup of the entire system - required to restore it b.e. in case of a harddisk-crash. Backup of my own system is about 1.7GB.
Note that you might need to restore the /dev Directory by hand - as the find-process makes some junk from time to time in there.