See in Disk management if the HDD is there and if it is, then check if it is online (right click on the left side icon in the lower right pane)
For clarification I believe you mean initialised not online

or you mean enabled, which is done through the properties of the device in disk management, or simply right clicking the HDD in Device Manager

How do i find that?
Right click 'My Computer' and select manage. From here you can access device and disk management.
Update : Are Hitachi HDP 725050GLA360 ATA device
And
Hitachi HDP S72168PLA380 ATA device
Different things?
Yes two different Hard Disks. First one I can work out the info but second one is not a valid serial number so I'm guessing at the info in it.
HDP 725050GLA360 = Hitachi DeskStar 7200rpm 500GB 16mb Buffer
HDP S72168PLA380 = Hitachi DeskStar 7200prm 80GB 8mb Buffer??
How to read the Deskstar model number
HDP725050GLA360 = 500 GB,16 MB buffer
H = Hitachi
D = Deskstar
P = Series prefix
72 = 7200 RPM
50 = Full capacity — 500 GB
50 = Capacity this model, 50 =500 GB
(40 = 400 GB, 32 = 320 GB, 25 = 250 GB)
G = Generation code
L = 26.1mm z-height
A3 = Interface, SATA 3Gb/s (AT = PATA)
6 = 16 MB buffer (8 = 8 MB)
0 = Reserved