This is something I wrote to help me mount and unmount USB storage on my HTC One S.
This requires a rooted device with USB OTG (On The Go) functionality at the kernel level (I think that this should be anything based on 4.0 or above but don't quote me on that!).
You also need the necessary cables... Plenty of advice on that if you Google around.
Please don't hit my for a 1 star review if it doesn't work on your phone. USB OTG is a bit flaky - especially on custom ROMs and non-stock versions of Android. Get in touch and let me know and there might be a work around.
Some devices automatically mount USB drives, and some work with the existing tools... mine did not... Hence this app.
You can change the mount location, media location and the regular expression used to match the USB drive in the media location.
These details can then be saved and remembered for future use.
Anyway - some may find this useful so enjoy.