I'm not sure if this has been resolved yet or not, but judging from the evidence given my initial thoughts were the HDD isn't receiving enough power to spin up.
There are a few things you can try:
- Test on every port on the laptop
- Do not connect the HDD through a USB hub
- Try a shorter USB cable (Longer and/or cheaper quality cables can result in a high enough voltage drop that the HDD can't spin up)
- Use a
dual-input USB cable so that the HDD is being supplied power from multiple USB ports
The fact that it was working previously for the last 5 months, as stated above, does suggest that the problem could lie elsewhere. However, inadequate power supply shouldn't be ruled out and can easily be tested to help isolate the problem.
If the above doesn't work, do what ohexploitable suggested above and download, burn then boot a linux live CD (most distributions will be adequate). If we can determine whether the HDD can be mounted using linux (on the laptop) we will be able to more accurately judge whether the problem lays in the hardware or software.
the virtual cd drive(of the WD HDD) is shown in disk management and it has been assigned a drive letter..
I don't know much about the software WD use for their virtual CD drive, but it is weird that it is appearing in Disk Manager and the actual drive is not. Is the virtual CD drive appearing in My Computer? The virtual CD drive isn't assigned to a drive letter that is already in use is it?
To best isolate the problem perform everyone's mentioned tests and report back with as much information about the outcomes as possible.