If it doesn't yet have a drive letter, it can't show up in My Computer but it will show in Control Panel>Administrative Tools>Computer Management>Disk management. Right click it - it's probably Disk 1 as Disk 0 (zero) is your system disk - and you might see a message inviting you to initialise it. Accept that and let it run. If it doesn't offer that, click to Format and during that process, you can assign a drive letter or accept the one it gives you. Once all that is done, it will show in My Computer and you can make use of it.