One of the many reasons for a disk drive not showing up in Windows is that it hasn’t been allocated a drive letter. In systems that have removable drives like USB memory sticks, the drive letter is automatically allocated by Windows. If one of these drives is attached while you install a different drive, for some reason Windows either tries to allocate the drive letter that the USB already has, or doesn’t allocate one at all.
If the new drive appears in the Disk Management window, highlight it in the top pane right click, and select "Change drive letter and paths." Select an unused drive letter and click Ok. Windows should then populate the bottom pane with the new drive and it should appear. If the device is a new hard disk, it may need formatting to enable Windows to use it, but the Disk Management application will tell you that, and offer to format it for you.
If all that doesn’t work, then we need to dig a little deeper into the system. Open Device Manager through the Control Panel and see if the new drive is there. If it appears, right click on the drive in question and select uninstall.
Rebooting the machine should force Windows to rediscover the drive and hopefully install it correctly. If this doesn’t work, repeat the process and also uninstall the appropriate driver. For a hard drive it may be the secondary IDE controller, for USB it will be the USB controller and so on.
Reboot the machine again and force Windows to reload all the necessary drivers and rediscover the drive.
If the drive still doesn’t show up after following all these steps then you need to read my article on troubleshooting “No Disk Drives in Device Manager."