Have you tried resetting the board, e.g. removing the batteries for several seconds and then putting them back in? Also, make sure they're fully charged when you attempt any change on the sound fonts!
Other possible sources of error are a faulty USB cord or the USB port used. No USB 3, especially in combination with Win 10 - the boards don't like that
USB 2 is fine.
A general step-by-step guide for uploading the default font (or any font, actually) is available here:
http://www.saberforum.com/index.php?topic=28888.0