The rule with back up was... Back up as often as your NOT willing to lose data. You have two current backups, and a working copy? I'd say if you add a new 100.00 backup every two years. You'd be only two years at the oldest. Maybe close to 1/3 life of the drive spec. That is pretty bullet proof, if you need ALL the data. 4T of music files, Thats a lot of music..
If the grid went down, could you still play the music? I've got a working Victrola and plenty of 78s. What do you think? Say a nasty Solar Flare. Could you still play music? All the digital stuff gets smoked.
WE'RE civilized man, we have music... WE WILL SURVIVE..
Just no, Yoko Ono. ;-)
Regards