I'm still not clear as to whether the machine in question came with Windows 10 Home "out of the box." If it did, and only if it did, it should be entirely possible to do a completely clean reinstall without need for a license key, at least to get the system up and running.
I agree that the HDD should be disconnected during the completely clean (re)install under any circumstances.
If the machine has a legit license for Home, one of the beauties of Windows 10 is that all Pro features are included, but not activated, as part of the install of Home. If you then acquire a Pro license key, you can simply go into settings, Update & Security, Activation Pane, and enter the Pro Key there. It will then go through the process of activating those features without any need for a reinstall of Windows 10 itself. [That's how the machine I'm typing from got updated from Home to Pro.]
