This is the response that I have got from FiiO CS
Thank you for your mail and support to FiiO!
Regarding to your problem, we suggest you use a tool to press your E18's reset hole then change another usb otg cable to have a try. Pls make sure each connection is good contact.
Or you can purchase an USB OTG cable, then connect the supplied usb cable to the USB OTG cable to realize another micro-micro USB cable, like the attached picture.
We suggest you use the built-in player. Pls try to move the bottom switch to CHG then move it back to "Phone in" to have a try.
In theory, the E18 can compatible with HTC one m8.
In theory lol...