I have tried many times now to register my POD HD500 and I keep getting the same message, "ESN could not be verified. Please confirm the supplied USN. ( Error: 523)". I have tried just about everything (upper case, lower case, with and without parenthesis, replacing 0 with O) and I'm starting to wish I never bought the thing, I have searched the net and there are many posts regarding this and nobody seems to have an answer. Please help!