I have been backing up my 2tb WD MyBook Essential to 2 1tb MyBooks (all USB). I have purchased a second 2tb MyBook. Win7 64-bit does not see the 2 2tb drives at the same time. I believe the BIOS is seeing them. I can mount either 2tb drives separately, but not together. They do not show up in Explorer. They do both show up for dismounting in the Hardware dismount utility Win7 has in the Notification area of the taskbar. I'm using an Acer Aspire 5536-5142. Suggestions for getting both 2tb drives running at once solicited. This is a home setup - not commercial.
OK. The problem is solved, but not all of the mystery. In the Win7 DiskManagement (put diskmgmt.msc in Start Menu search box), both drives show up, but the second one to be connected says 'Offline'. A mouse-over of 'Offline' elicits the msg: "This disk is offline because it has a signature collision with another disk." I do not know what this means, but I know the immediate solution. Right-click on the word 'Offline' and select 'Online' from the popup menu. The drive will be assigned a letter and will now function normally within Windows. Another article I read states (unofficially) that you cannot assign the drive a letter in this situation: you must allow Windows to do it.
The drives perform completely normally. After restarting Windows, the drives mounted ok. I do not need to set them to 'Online' in the disk management utility after each boot, as I had feared I might.
I have been searching for how to change the drive signature, but I have yet to find a way. Luckily, it doesn't seem to be necessary.
BTW - the hard format of my 2tb USB 2.0 drive was about 28 hours. It did not help with this issue.