best software Koreader and best people here, I am a kobo/koreader fan now.
Since I successfully installed a 2 years old Touch. I bought a mini and Aura recently.
Touch: changed internal microSD to 16G ($49 two years before, bestbuy)
Mini: extended internal microSD to its 4G capacity ($39 Factory Direct, Jan 2015)
Aura: did nothing, since it's internal card is not replaceable ($99 Christmas sale, Dec 2014)
this link helps me to replace/extend internal card
https://www.mobileread.com/forums/sho...47&postcount=1
Thanks again, the software developer and the best people in this forum.