I have ipad air 16GB and I need some WI-FI solution, because I'm running out of HDD space. It has to be wifi, (hopefully with internal battery) so it can be just sitting near by and used as storage of sound banks, midi files, sort of "extended HDD". It has to be able to transfer audio-wav, aiff files between itself and ipad. (some don't....video only) Some of these units come with a downloadable free app and some don't. What can I use? Is the app necessary, will ipad see the external storage? What do you recommend a card reader or HDD? Thanks for clarifications and help.