External drives not being recognized

I have two external WD Drives connected to my computer. Lately (and now) only one of the drives are recognized. When I initially hook the drives up, I get the option box that asks me if I want to view the files so I believe the drive is alright and I can here the disk spinning. What is causing my computer to recognize only one of the drives?
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noxchoConnect With a Mentor Global Support CoordinatorCommented:
One of the drive could have errors in file system. In Disk Management right click in the drive and select Add\Remove Drive Letter. Select the letter and assign it.
After that does it show in My Computer?
If yes then remove the drive and connect it again - does it now get the drive letter?
Thomas GrassiSystems AdministratorCommented:
How are the drives connected? USB Firewire?

If usb can you plug one at a time in to see if any thing changes

What errors in the event log appear?
nobusConnect With a Mentor Commented:
test them on another PC, this will show if there is a problem with the drive, or connection, or PC

also - try connecting them to the USB ports near the keyboard/mouse connector
and alternatively, use a POWERED usb hub like this :  http://www.belkin.com/IWCatProductPage.process?Product_Id=158910      

note that USB power is MAX 5 V at 0.5 A = 2.5 W, and not all ports provide that (you can check by connecting the drive to a port, and look it up in device manager, in its energy tab
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noxchoGlobal Support CoordinatorCommented:
Connect both drives - right click on My Computer - Manage - Disk Management.
Does it show both drives there?
If it does - do they both have drive letters?
brightfarmAuthor Commented:
My Computer - Manage - Disk Management shows both drives but only one of the external drives has a letter. The drives are USB connected. I tried connecting the drives individually and it made no difference. I don't know how to view the event log.
can you assign a drive letter then? that can be all it needs
right click the disk and select change drive letter
for event viewer, click start>rightclick my computer, then select manage
in the left pane , select logbooks, and then windows logbooks
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