Resolved Problem with External Hard Drive Recognition

I recently got 2 4TB Western Digital external hard drives.
I transferred some files to each drive. The problem is that when both drives are plugged in simultaneously my computer can only seem to recognize one of these drives at a time.
When I plug either one of them in but not the other, they are recognized as "Drive M" but if I have them both plugged in then only one of them is recognized as Drive M.

In other words, the drives work fine so long as only one of them is plugged in but not the other. Which means, if I wanted to directly transfer files from one drive to another, I won't be able to do that.

I went to "Devices and Printers" in the control panel and noticed a troubleshoot icon next to my-PC icon. It says I'm missing the driver: microsoft teredo tunneling adapter. But when I clicked "Apply this fix" the driver could not be installed.

 

I plugged them both in and went to Disk Management.

One of the drives is offline. It says "The disk is offline because it has a signature collision with another disk that is online ". No idea what that means.

 

Try reading this post by jabshire84http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/after-installing-a-second-wd-1tb-my-book-drive/28fe3c13-713a-449c-ac4d-4f22d9f70f8e

This is horrible advise unless you know the drive is empty. Use the Windows Diskpart command. Select the Offline Disk. Enter the command Online Disk. Diskpart is much more powerful than the Disk Management. I had this problem and Diskpart brought the drive online with NO issues where Disk Manager continued reporting a Signature Collision.
Sorry to sound harsh but to format the disk? I would think removing the disk and doing an Import Foreign Media would probably work allowing you to then place the disk bakc into the original computer. If you still get Signature Collision...use the above it works fine.
My Signature collision was cause by Acronis Disk Suite...HORRIBLE software if you have Windows 7....have had noting but problems...seems any disk it touches corrupts the MBR and Signaure area.
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I had this exact same problem and I used this answer to help me. In my case I left a second hard disk inside my drive when I rebuilt my primary disk with Windows 7. The second drive wouldn't be seen in Windows Explorer, Disk Management showed me the disk was present but "Offline due to Signature Collision" - there wasn't an option to make the disk go online via right click either.
I took the second disk to work, put it into a caddy and moved all the information off of it and then ran a full format. Came back home and plugged it in but no joy. I couldn't right click and make the disk go "online" again through Disk Management as before. I ran across this answer through google and as a thank you will write up the process.
If you have the same problem of Signature Collision try to go to "Manage Computer" and then "Disk Management ", find your drive and then right click to make it go "online ". If it wont work then go to a command prompt and type the following in " ", hit return to move through:

"Disk Part "

(this brings up a separate window in Win 7)
"List Disk "
(This will show you which disks can by physically seen, offline and online)
"select disk X "
(Where x is the number of the disk you want to make online)
"online disk "
(This should bring you a message saying your disk is now online)
"Exit "
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Thanks. Everything is fine.

I went to Disk Management, right clicked on the offline drive and clicked "online" So now I have Drive M like before and the one that was offline is now Drive K.

Thanks a million.