.ICBU File Extension

Calendar archive created by iCal, a calendar application included with Mac OS X; contains a backup of one or more calendars; used for restoring calendars in the event data is lost.

ICBU files can be created by selecting File → Export... → iCal Archive... in iCal. The export format is actually a folder (with the ".icbu" extension) that contains a subfolder for each calendar and .ICS files for the calendar events.

NOTE: When restoring backups, iCal overwrites all all existing calendar data. The program prompts the user to overwrite the data before performing the operation.