If it doesnt have a drive letter then you wont be able to access it.....
What happens when a HDD is connected, is that Windows checks that there are drivers for the device and if not gets them.
Then what you need to do is to format the device using disk management then when that is done , you then right click and activate the disk, where it gets assigned a drive letter and can be accessed through my computer
Job done.
If the above isnt happening then you have an issue, somewhere along the line..