One thing to be aware of if you are going to try any of those expansion type drives...
My first thought.. which it looks may be the same on the Sandisk one that was linked earlier in the thread..
By default.. the only thing the iOS would likely be able to see On the device, is the pictures/video. It probably runs then the same as like lightning to card reader cable. Really only for transferring picture/video files.
The sandisk does appear to allow to play audio.. but through ITS software
Generally I do like/support the apple environment.
Sure you have to load your music through iTunes.. but other than the small amount I do carry on my phone, my BIG collection stays at home. The iTunes + mobile + airplay capable devices, give some unlimited choices in terms of streaming music fairly easily while at home, without having to always have the music ON you. (one thing I have yet to quite get with android. CLOSER with the chromecast audio.. but still then reliant on 3rd party DLNA streaming, etc).
But the lack of other connectivity, even for viewing/storage.. is the biggest downside.
I would be completely fine with a lightning to USB adapter. Though, likely that would INSANELY drain the power on the phone though I would guess. It would likely have to be a FORCE power added one.. where you need to plug in the power charging cable as well into it (to then power the USB stick)