check ethernet connection

Yesterday I had to remove all cables from my Mac Pro (3.1) OSX 10.9.5 in order to install 2 cards.

After I connected the cables, the Ethernet connection to my Drobo did not switch the Drobo on.

I testet the cable, it works fine. When I went to System Preference>Network, it showed me, that there is no connection.

I set from the location to automatic, set DHPC to manual address, it always says: Cable unplugged.

I tried an other cable, both ports give me the same message.

I updated the Java drivers and the security update from Apple.

I can run the Drobo via USB, but it is extremely slow.

I need every available speed for video transcoding.

How can I check, if the Ethernet cable is working?

I don't want to fiddle too much with the cards, one I borrowed and is very expensive.

Thank you!