If you dont have any data stored to it yet... go to Disk Management and you should see it there possibly with no partition created. Create a new partition and format the partition ( which usually happens after the partition creation by windows ). Now a drive letter will be automatically assigned and you should use it without any problems.
***Note: If this is a smaller 2.5" drive that is powered off of the same USB cable that it communicates on you could be dealing with insuficient power from the USB port to the drive. If this is the case, you need to add a powered USB HUB or if this drive has a way to power it from wall power, use the power adapter that came with it to power it vs using USB power source. I have seen some laptops unable to power external 2.5" drives and require a powered USB HUB or wall power via the power adapter that came with it optional.