You might be confused a bit…
There are differences between backing up to an external drive vs a NAS.
MBL is a NAS.
arrangements? Well it all depends on how you do backups. But none by default from MBL. MBL Duo has redundancy.
Some reading:
http://howto.cnet.com/8301-33088_39-57556308/digital-storage-basics-part-3-backup-vs-redundancy/
http://icanhelpyourmac.com/tip_archive/understanding_apples_time_machine/index.html
