Hi and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, the USB port in its current/factory default condition does not support the connection of an external drive. With a DCH6416, an external drive would have to be connected to the eSATA port. This is as stipulated in p. 6 of the Motorola External DVR Storage User Guide available HERE.
I realize that the above information may perhaps be not what was expected. Unfortunately, there is no quick-n-easy fix and neither is there a workaround; sorry for having to be the bearer of bad news.