Yeah I understood the backup part. I wonder if there's such a thing as an sd card duplicator.
Edit: just remembered my phone has an sd card slot. So you'd just need an OTG adaptor cable, sd card reader (not sure if you'd need a powered hub for it)*, and (maybe) file mgt app (I use File Manager HD).
Swap in new card(s) as it/they fill up.
*or one of
theseEDITED: 28 Feb 2017 16:36 by DSMITHHFX