How many USB-powered 1TB can you plug into an HP G71 Notebook

Question

I'm using Windows 7 (service pack 1).  I have 3 USB ports, one of which I've extended with a 4-slot extension. 
I bought a Western Digital Elements and an I Omega 1 TB USB portable hard drive (each nominally clocks in at 930 gigabytes).  I plugged one into one side of the laptop and another into the 4-slot USB port extension. 
These worked great, except I filled 'em up too fast.  So I needed another one.  I bought another Western Digital Elements drive, exactly like the first one.
The laptop will make a sound when I plug in the third hard drive ... a positive sound that it makes when I plug in the other two.  However, the window that pops up for the other two does not pop up for this third.  Also, the USB port extension begins chiming ... just a little sound, but it's annoying, and doesn't stop until I unplug the drive.  Additionally, the third hard drive is not assigned a drive letter in Windows Explorer.  I can, though, unplug the first WD drive and plug in the second WD and the laptop will recognize the drive and assign it a drive letter.
In short, I cannot have all three of the drives up and running at the same time and visible in Windows Explorer, which would allow me to switch files between them.
Is there anyway I can get all three visible and working?  Or is there something wrong with the new WD Elements drive?  (I think the drive is okay.  I copied files to it and used those files just fine).
Any help would be appreciated.
Thanks.

Answer 1

Sorry ... none of that exists on my drive.  In fact, when looking for a portable hard drive, I specifically sought out drives that had nothing but open space on them.  The I-Omega and the Western Digital Element were the two that I found that met that criteria.  Nothing in the very thin documentation even suggested that there were hidden files let alone program files for backing up, or partitioning, or whatever.
As for the Disk Management screen ... Yes.  I have that.  I click on "Computer" in the "Start" menu.  However, as you can see (if I've embedded the jpg correctly), even with the second WD drive plugged in, you cannot see it in this menu.
The only place you where you can tell it the computer "sees" it is when I click on the "Remove Hardware Safely" icon.  All three connected portable hard drives are listed in that menu, but no drive letter is associated with the second WD.  Also, in this screen there is only one thing you can do and that's to disconnect one of the drives.  I've tried to right mouse click and get a drop down menu, but no luck.
Okay ... I was wrong ... you can see it under "Devices," but the second WD is a little different.  The I-Omega and the first WD get a tool bar that lists Add a Device, Add a Printer, Autoplay, Browse Files, and Eject.  The second WD -- as you can see -- does not get the Autoplay and Browse options.  Also, I've tried "Add a Device" and the computer did not find the drive.  Oh, by the way, when right clicking the second WD for Properties the pop up menu says that the drive is working properly.
So ... you can see my frustration and confusion.  I'm beginning to wonder if the computer simply doesn't like the fact that I have two WD drives that are exactly the same.
Hope there are some other ideas to try ....
Thanks