Nav Data is loaded using a portable data loader (PDL) or an airborne data loader (ADL) from one or more standard (but differently formatted) 1.44 floppies.
The PDL is a portable unit which is plugged into the a/c system and the data transfered.
The ADL is a floppy drive directly wired into the a/c which does the same job as the PDL.
Each database is valid for 28 days and a new database is loaded into the FMC several days before the current one is due to expire. So the database should always be valid. However in the event that the data base can not be loaded or is lost in the post
then there is provision in the MEL to operate with an expired database. The crew will cross check waypoints and other data with their 'Jepps' charts to ensure currency.
Hope this helps,
Cheers.