[iknowsquat]

@blockhead, find a post of mine with my Sig. There's a link to xda to fix SD & USB issues. This is just a guess but there might be some info there. That thread is partly about the usb not charging at all which is not your issue but its worth a shot.
fwiw-for me, Google did not turn up anything on "htc evo charge light staying on" which could mean its isolated to your phone.
Good luck.