Two 4.0TB My Book at the same time ,Widows Only Recognize one at the time!?

[ashkekabir]

sorry i found the solution .it was disk colision something like that 

for those who may have the same problem you must go to my computer → disk managment ->fhttps://wdc.custhelp.com/app/answers/detail/a_id/6571

follow the instraction on page above