If you are using one un-partioned HDD then even the HDD is powered up, you will not see any drive letter as there is no usable partion in it...
Do one thing, Go to Disk Management....
by Right Clicking to
My Computer, then Click
Manage...
In the computer management console, please use the left navigation to go to
Disk Management
Now in there if you see unallocated space as in Black, then please format that as per you need or if u see active partion but without any drive letter then please asign a drive letter to use it under windows, by right clicking up on them...
thank you...