Windows 7 - Win 7 doesn't assign drive letter with 2nd Western Dig Elements drive

I have Win 7 home premium on an HP laptop with 4 usb ports. I have 2 Western Digital Elements 1.5 TB drives for backups. If I plug in one of the drives, Win assigns a drive letter, but if I add the second one it doesn't.

To troubleshoot, I've plugged each drive into every usb port on the laptop, and each port reads each drive alone, but if I plug in the second drive while the first is still in, it shoes up in devices but does not get a drive letter. I tried assigning a different drive letter to the device plugged in first, but Win still doesn't assign a letter to the second drive.

Any advice? Thanks!

Did you try to plug them in reverse order (the second drive first and when that's recognized, plug in the first one)
My drives are WD My Book 1TB and 2TB. at first they refused to play together nicely.
I had to run the WD-DriveManager (which came with the 1TB drive) once to " beat" them into submission

Good Luck
-DG