In case someone else has this problem, I got a brand new Mac Pro with two ethernet ports and both didn't recognize the ethernet cable that work with two other computers (one Mac, one PC). In System Preferences > Network it said that "No cable was connected". Note that I use the German version and don't know what the exact terms are, that OS X uses in the English version. Also I'm not sure if it works for all Apple computers, it worked for the Mac Pro. Anyway, I called Apple and they tought me this trick: 1. Common sence (check if the cable is ok, if you have another computer check the cable with that one, restart etc.). Also check under About This Mac > More information > Network (click on the header "Network") if Ethernet 1 and Ethernet 2 are listed (in case you have two, like the Mac Pro does, otherwise just 1). If no, that migh be your problem, didn't have that one, if no, go on with step 2: 2. Turn the computer off and disconnect all cables (including power and USB/FireWire devices) 3. Hold down the On/Off-switch on the front side of the tower for about 20 seconds, still all cables are disconnected. 4. Reconnect the cables except for the Ethernet cable. Restart the machine and from the start hold down these four keys on your keyboard at the same time, until the Start sound (heavenly chime) is played for the third time. Then release and wait for OS X to launch (in my case it did a 4th chime, doesn't matter). Command (Cmd, Apple-key) Option (Opt, Alt) P (as in Peter) R (as in Richard) Step 3 and 4 discharge and reset something (forgot what exactly). 5. After OS X has launched, reconnect the Ethernet cable. Now check again, where it said something like "No cable connected" (I use the German version where it said "Kein Kabel verbunden") it should now say "No connection" or there should be a connection, depending on what your settings are. Either way, the cable should work now.