My Book Essential 3TB is detected only after powering down

 I own two My Book Essential 3TB external HDs. One is just working fine. My 2nd HD has weird behavior. Every night, I disconnect my HDs from my Windows 7 64 bit laptop. In the morning when I connect back both HDs, only one HD is detected. 2nd HD has white blinking LED on it. 2nd HD is detected in my laptop only after I disconnect it from the power and reconnect it. 

I am just afraid that it will die soon. Please help me to identify the problem.

Hi, that sounds like a disk signature collisions, connect the two my books to the computer, open the disk management window, right click on the one that shows offline and select the online option. Check the link below for the steps to access the disk management. If that doesnt work run a test with DLG.