[ubuntu] Wired Connection Disconnected always, but it works fine

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http://ubuntuforums.org – hi everyone, I am having trouble with the wired connection... I have installed ubuntu 12.04 before. I used the pc with Win XP, now i removed the hard disk with win xp installed and replace it with another hard disk with ubuntu 12.04. The problem is that i cannot connect to the wired network. Always disconnected... I tried the solutions mentioned, but nothing works. I plug the network to my laptop, it works fine. but if i plug it in desktop, it's always disconnected... can anyone help me fix this??? I would appreciate it well. I will make this desktop as my server for the new system i have c (Hardware)