I got a new WD-1TB external usb hard drive. It is the "My Passport" series. When I plug it in I get the familiar tones and it says installing drivers was a success. However there is no drive letter for it in windows explorer so I cant access it. I have Win-7. I have installed a lot of external hard drives and never had a problem before. I tried several usb ports with same result. I cant remember exactly where I was or how I got there but in my troubleshooting I found it and it said this device is operating properly so I think I can assume its not dead. Also the light is blinking slowly which is a good sign.
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EDIT: I got it working by following the procedure of a YouTube video from a fellow who had the same problem.
Also I want to add that it worked in my Win-10 computer downstairs.

What a strange problem.
I found several YouTube videos about "fixing" it by manually assigning a drive letter using the Disk Management Tool. For example,
https://www.youtube.com/watch?v=pjotE0sFegQ

If anyone else has the problem, the fastest way to open the Disk Management Tool is to run
diskmgmt.msc

You may have to run diskmgmt.msc as Administrator.
If you don't, the option to Add or change the drive letter may be grayed out.

You can also run
compmgmt.msc
and click where it says Disk Management.

Then right-click the volume of the external drive, click the Add button, and assign your drive letter.

What the videos didn't explain was why this happens.
It sounds like a Windows 10 "feature" (aka bug) that got backported
to Windows 7.

That is the exact video I followed!

So, it worked on your Windows 10 computer, but not your Windows 7? THAT is stange... I wouldn't be surprised if it was the other way around, but am that it was Windows 7. I have the same EHD and didn't have any problems at all on my Windows 10 computer,

Correct, worked on 10 but not 7.