How to completely wipe a hard drive clean

[Toastey]

So my friend let me borrow a flash drive that he said had a copy of Windows 10 on it, and he let me borrow it for my latest build. However, now it is saying that I cannot activate windows 10 because it is not a "genuine copy of Windows 10". I'm not sure what that means, but it doesn't sound good. So what I want to do now is completely wipe this hard drive clean, so I can just download a copy of Windows 10 that I'm going to buy. I don't want anything left on the hard drive. I think it's its called "factory resetting", but I'm not sure.

 

[Calvin7]

When you go to install Windows, at the first screen you have an opportunity to choose where to install. Choose the drive and proceed to delete all existing partitions on the drive.

 

[JoeMomma]

The "not a genuine copy of Windows 10" message, just means your license key is not valid.
You could input a 'real' serial number, but it's probably a pirated copy of Windows and it is best to wipe it.
Who knows what else is in there?

There is the easy way (recommended) and the overkill way.
The easy way is download Windows 10 to a flash drive with a valid key first.
When you reinstall Windows wipe the partition as the first step of the install process.

The overkill way is to use dban. Darik's Boot And Nuke uses a military grade Department of Defense drive wiper.
It will write zero's to every bit on the drive. Multiple times if you want. It takes a long time.