Firstly, I seem to remember that this Drive is USB Powered.
It is quite likely that the USB Ports of your Laptop do not deliver sufficient power to run the Drive. You may need something like this
On the point of assignation of a Drive letter. The PC/Laptop will assign it a suitable Letter every time it is plugged in.There is no reason that this should be the same every time. It will depend on the number of USB Devices you have plugged in already.