july 2018 update:
this update relates mostly to us that have newer iOS devices that lack compatibility with some of the devices that have been covered previously in this thread. currently I have an iPhone X on iOS 11.4. here's what works for me:
the SD/micro SD card reader I use is branded WECODO and uses an app called iDiskk Pro. it also has a USB and micro USB head for android users. the listing is no longer live on amazon.com but a quick search for iDiskk comes up with a few readers that likely use the same app, and if so, will likely work with newer iOS devices and OSs.
perhaps this ~$15 device that's actually listed as iDiskk branded is the new form factor.
anyway, here's what it looks like in action.
please let me know if you have any questions and I'll try my best to give an accurate, timely response.