I have recently purchased a used Precision T5600. The front USB3 socket was damaged when I received the unit (the blue plastic part that the pins are on was broken) so a USB plug could not be inserted. Windows 10 also complained about a power surge on the USB port. I disconnected the USB3 cable from the motherboard header and the error didn't return so I am guessing there was a short in the USB3 socket.
I ordered a replacement front I/O panel that came with the USB3 cable which I have now installed (an absolute pain to replace as it needs the motherboard to come out to route the USB3 motherboard cable behind it...). Anyway it didn't work - either from within Windows or on boot. Windows also now shows the Power surge error message when a USB3 device (portable hard drive) is connected. The new USB3 motherboard cable has the 20-pin plug for the motherboard header and an angled USB socket that fits into the front I/O panel, however it is black plastic inside as opposed to blue like a normal USB3 socket. I have checked out other Precision workstations with the same chassis and they all appear to have 1 x USB3 and 3 x USB2 sockets so I am guessing they are all the same? Plus its a 20-pin USB3 header on the motherboard so even if the cable was wired as a USB2 socket (i.e. just using the USB 2.0 data pins on the USB3 header rather than the SuperSpeed RX & TX pins) then the socket would work as a USB2 socket?
I am tempted to buy a standard USB3 motherboard 20-pin plug to USB3 socket to test it as the Dell cables are a bit rare and rather expensive just to test with. Is the USB3 motherboard header standard on the T5600? Any other ideas on what could be wrong?