Serial cable? It has no serial port! Are you connecting it via a network to serial-cable, that will not work.
The RJ45 Ethernet port is only a network port, not a serial-port.
You are suppose to use a MicroUSB of mini b type (note Micro-USB, not Mini-USB) data cable for flashing.
The CuBox is supposedly unbrickable so you just need to flash it corectly via USB (again MicroUSB).
Oh, and do make sure it has power too, as the Cubox can not be powered over USB.
-USB, not "Mini
-USB", they are not the same, that is why. Micro-USB is a newer standard:http://en.wikipedia.org/wiki/MicroUSB#M ... connectors
Different usb connectors. From left to right: male Micro USB B-Type, proprietary (not USB), male Mini USB (5-pin) B-type, female A-type, male A-type, male B-type. Shown with a centimeter ruler. Female A-type connector (4th from left) is "upside down" to show the pins.
So what you need to go out and buy is a "Micro USB B-Type" / "Micro B USB" cable, good to have
MicroUSB may not be as common as MiniUSB, but MicroUSB is the official standard for mobile devices.