Limit on number of external hard drives?

Does Windows 7 have a limit on how many USB external hard drives it will use? We have 5 WD My Book 1TB drives; 4 of them are recognized and work just fine with Windows 7 Home Premium 64 (which we recently did a clean install of to replace Vista Home Premium 64), but the 5th one doesn't show up with a drive letter (in Computer or Windows Explorer), even though it does show up in Control Panel. The one we are having trouble with has had the latest firmware and WD SmartWare installed, and our computer has the most current Dell BIOS installed. We know the drive works, because we can plug it into our other computer which has Windows XP on it and it comes up with a drive letter just fine. Just wondering if there's a limit on how many of these you can use with Windows 7, since we had no problem with the drive until we installed the new OS.

What happens is, any 4 of the drives will work, but not the 5th one. Let's say I have drives A, B, C, and D connected and working fine, and drive E is connected but not working; it shows up in Control Panel, but nowhere else. If I disconnect, say, drive A, and have drives B, C, D, and E connected, all four of them will work fine and show up everywhere. But the minute I connect the 5th drive--no matter which of the 5 it is--the 5th drive only shows up in Control Panel (and Windows makes a little sound when I connect it, so I know it recognizes it is there), but it doesn't show up in Computer or Windows Explorer with a drive letter.

Here's something interesting we just found out, after doing some more troubleshooting. All the drives I mentioned above are 1TB WD My Book Essential external drives. Just to see what would happen, we left connected the 4 1TB drives that were working normally, then tried also plugging in a 500GB My Passport drive--and it recognized it and gave it a drive letter! We actually had 5 drives connected, working, and recognized all at the same time. Then, we disconnected the My Passport, and plugged in a 500GB My Book Essential that we had around--and it was recognized, too! Again, 5 drives working at once. So the issue isn't that 5 drives can't work at the same time--could it be that when we get to 5TB of memory, that's the problem?

The drive may not be initialized. If you can see it in Disk Management, you can initialize it there and assign a drive letter.

I believe it must be intialized, because it is recognized normally if any one of our other 4 1TB external drives is disconnected. Any 4 of our 5 1TB external drives get recognized and assigned a drive letter automatically, but when we try to add the 5th one, the 5th one shows up in the Device Manager, but nowhere else (no drive letter is assigned).

Try to right click on it in Disk Management (if you can see it there) and assign a drive letter (right click on the right part of it in DM - not where it says Drive 1, or 2, or n)

Figured it out and got it working! In Disk Management, it showed up at the very bottom, but when I right-clicked on it, all the choices were grayed out. Then I noticed it was OFFLINE! It said something about a "signature" conflicting with another drive somehow. So I changed it to be online, then suddenly it was automatically assigned a drive letter and showed up everywhere--Computer, Windows Explorer, etc. It would never have occurred to me that it was offline! Thanks so much, WHS, for sending me to Disk Management, where I finally figured out the solution to get it working!

Sure, glad you got it figured out.