Hi there Motakuji,
That is unpleasant.
I would totally agree with X79. You need to double check the connection. You can attach it to another port and make sure that it is tightly fastened. In case you have spare cables, you can even attach it with different SATA and power ones.
Another thing you can do is to test it with WD's Data Lifeguard Diagnostic tool(both short and extended tests):
http://products.wdc.com/support/kb.ashx?id=PbN2Dk
If there is something wrong with the drive, you need to back up the data stored on it as soon as possible.
Let us know how this goes,
D_Know_WD