I've been programming an older USM-810 (by older I mean it's been used) and never had a problem with it. I unplugged it to move it over to my amp rack and made the mistake of leaving it plugged into the RS-232 port. Plugging it back in, it goes through it's normal boot cycle. The display gets to "sh3" and immediately switches to E13, before resetting and trying the whole thing again (and again and again and again).
I cannot connect to it using IQ (except once when I unplugged it then plugged it back in, and it immediately lost the connection when the thing rebooted).
Figuring it might be a bad power problem I switched circuits and had no success.