I see this a lot with USB drives. I found these general USB drive troubleshooting tips on the Microsoft support boards.
1. Access Disk Management and see if the USB device is listed. If so, and there’s no drive letter assigned, see if you can assign a drive letter to the device.
2. If the USB device is listed in Disk Management with an assigned drive letter, right-click on its listing and select Explore from the submenu. Hopefully, Windows Explorer will open and the device will be listed.
3. Connect the USB device *directly* to a USB port on the computer, not via a USB hub. Try different USB ports if your computer has more than one.
4. Do not use a USB extension cable.
5. Try alternate powering on/off methods. If the USB device contains its own power supply, try booting up with its power on, then try powering on only * after* the system has booted to a Desktop.
6. Try a different USB cable.
7. In the USB controllers section of Device Manager, uninstall all the USB controllers listed and reboot.
8. If the device in question is HD which you installed in a USB enclosure, jumper the USB external HD as Master (or Single if the HD is a Western Digital disk). A number of users have reported that jumper configuration corrected their non-recognition problem. In my own experience it didn’t seem to matter how a USB external HD was jumpered. But it may be worth a try.
9. If the device in question is a USB external HD, first check out the HD with the hard drive manufacturer’s diagnostic utility. If it checks out OK, and you can remove the HD from its enclosure (without voiding any applicable warranty), do so and install the HD as an internal HD to determine if there are problems with the drive.
10. If the USB device is connected to a USB 2.0 PCI card, try changing the PCI slot to which it’s currently connected to another one.
11. Access the website of the manufacturer of the USB device to determine if there’s any firmware update or info re the problem you’re experiencing or there’s any possibility that the USB enclosure itself might be defective.