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.
source: callforhelptv.com
Tech Help 
The drive may also have to be formatted before Windows Explorer can see it; that is what did the trick for me. This can be done through Computer Management -> Disk Management. Thanks!
This is very helpful. I just fixed this problem according to the second suggestion. I was unable to see my USB disk in Explorer.
Thanks!
Thanks! Suggestion #1 worked for me. I just purchased a Western Digital Essential 250GB and formatted it to NTFS, made partition active but it didn’t show up in windows explorer. The assign drive letter option in disk management says something about making it active via drive letter… so i guess WD passport is happier if you assign a drive letter. Excellent page btw.
I run win xp pro with a maxor 40gb drive loade with xp pro sp3.
I added a sata raid 2 port pci host card and then installed a seagate 250gb serial ata drive.
My mb bios does not see the sata but there is a bios chip on the card and the mator with xp can see it in admin tools>computer management>disk management as a ntfs healty and active. I did a quick format from the same tool.
I can’t figure out how to load a fresh version of xp pro on the sata drive. What I really want to do is make the sata my bootable with the maxtor as the slave.
I know the sata can’t be a slave so I am looking for any help.
Tom
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HELP please….
I got myself a Merlin 1000gig External USB HDD, it was working, until I formatted the whole drive, the drive still works, however i cannot “see it or access” it from my pc. Please someone help me out here.
1)I have tried to update the USB drivers.
2)I have tried a different usb port nothing seems to go.
3)I have looked under My Computer under manage, nothing there……
PLEASE GUYS help me out……
I have just built two of my older hard disks (one S-ATA, the other IDE) into external enclosures and have tried connecting them to two of my machines (one XP Pro SP2 and the other Vista), but am unable to ‘see’ either of them. I do not see them in Disk Management either despite the usual pop up when I first connected them indicating that new USB devices had been detected and were ready to use. Could it be the fact that both disks have multiple partitions?
Thanks very much i was wondering why my hard drive wasn’t showing up untill i found this website.
why can’t use for readyboost HPv165w usb devices