Default BackUps on external drives
Although I havent actually backed up to and external drives ,I usually move all my backups to an external drive ,the reason is this , if something gets corrupted I can always take out the sd card and reformat it , then transfer a backup onto the sd card (of course if recovery isnt there you can use Euclid Brothers recovery tool ,to get it back ! But this way I can keep tons of back ups without using my sd card 16 gigs!
Custom built pc 3.4 GHZ
Snow Linux 4 | MATE

~Kindle Fire 1st Gen~
[ROM][4.2.2] Chameleon OS [OFFICIAL][&Nightlies]|FFF | TWRP

~Galaxy Note LTE SGH-I717 - 16GB~
[ROM][JB] [4.2.2] Avatar ROM for Samsung Galaxy Note(quincyatt)|Team Win Recovery Project 2.3.3.0

~Samsung Galaxy S II SGH-I777 - 16GB~
[RoM][XWLSN-4.1.2-JB][SHOstock3 v3.0] [04/02/13]["Recovery Edition"]

~Acer Iconia A500~
[Dev]A500 JB-MR1(4.2) device building - v8 |recovery-thor2002ro-rev1.7.3