Unmountable Boot Error After Writing Zeros to Disc

Dell laptop with a WD 750GB Black hard disc. Decided to do a clean install of the OS after upgrading from Windows 7 to Windows 10. I wanted to go back to 7 and start with a clean drive and used Western Digital's Data Lifeguard Diagnostics for DOS.
Wrote zeros to the drive, and then booted to my retail version of Windows 7 Pro 64 bit, only to receive an unmountable boot volume error. Ran a quick test on the disc and it came back error free. Now running the full test but it takes about 90 minutes.
In the meantime, any suggestions on what is wrong and how to correct it? Thank you.

Very strange. When I went to the MS software recovery link and tried to download the file, I received an error message telling me the software I'm trying to download doesn't support the chosen language. . .English.
I confirmed the product key and tried 4 different times on two different computers.
Is there another location where I can create a bootable USB for W7 Pro, SP1, 64 bit?

Just realized I was inputting the product key, not the COA. They aren't the same, I'm guessing? So where do I find this COA?
This is a Dell laptop that came with W7 Home Premium. I purchased a retail version of W7 Pro. No idea what the COA is.
Is there any other way to create a bootable USB? Right now I'm feeling !@#$%^&* Need to get my laptop up and running. Never thought it would turn into an all day and all night project.

It was purchased online a number of years ago, possibly from Newegg, but I don't recall.
In any event, strangely enough on what was about the 12th attempt to get the OS to install it suddenly did.
Don't know why it wouldn't, don't know why it finally did. But I'm relieved.