The Terris people in Mistborn store knowledge in metal bands they wear on their bodies, called copperminds. Knowledge is transferred into the coppermind as a true Second brain because any knowledge stored leaves their memory immediately, yet can be recalled as long as the metal band is worn.
Copperminds store more than an single mind can hold and represent a near perfect Personal knowledge management system.
There is no mechanism for backup. To transfer knowledge one Terris must read it to another who can then store what they hear.