Copy and pasted from Seagate KnowledgeBase (Answer By Xmustanguyx):
I just got my 2TB FreeAgent GOFlex Desk and plugged it into laptop.
Formatted it works fine. Plugged my 500GB FreeAgent GO and NADA.
Swapped just as you all did and nothing, both worked independently and
would not work together.
Here's what I did to fix:
My system: Gateway with WIN7 64BIT Home Premium.
Externals are USB.
Right click "My Computer" then click "Manage", then navigate to "Storage" -> "Disk Management". Find the external that shows a drive
letter and change it to something else. (I don't think this is
necessary but you can change it to what you like after you get them to
work. This is what I did to get it to work.)
Next, right-click the the drive that is not showing up, in my case I
changed the 500GB to N:\ and the disk showing was Disk 2 Basic1863.02
Right click the GRAY area in the right hand side. Just under the red
arrow that is pointing down.
Under that you will see the error message if you hover over the HELP.
It says, at least on mine:
Offline (The disk is offline because it has a signature collision with another disk that is online)
Now that you right-clicked, it selects "ONLINE" and this will bring it
online and automatically assign another drive letter. At this point
you can change drive letters or whatever else.
The post at Seagate is 3/4 down the page, but the whole page is on the subject.