Reinstalling Win 10 on new Hard Disk Drive

Hi crescent786, you can reinstall Windows 10 at any time, you will not need a Product Key, Activation will be automatic and it will not cost you anything !

Since you previously had Windows 10 installed and activated on that PC during the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 10 on this PC', and activation will not be an issue, your PC will have a digital entitlement stored on the Microsoft Activation Servers

Make sure you install the same Edition of Windows 10 (Home, Pro . . . etc.) you previously had installed to avoid Activation issues

Click this link:
https://www.microsoft.com/en-us/software-downlo...
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 10 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool

Then, Boot your PC from the Installation Media you just created (change Boot Order in your BIOS) to begin installing Windows 10

If you have problems booting from a boot disc, you may have UEFI BIOS:
Insert the Bootable Installation Media, then go into your BIOS and make the following changes:
1. Disable Secure Boot
2. Enable Legacy Boot
3. If Available enable CSM
4. If Required enable USB Boot
5. Move the device with the bootable disc to the top of the boot order
6. Save BIOS changes, restart your System and it should boot from the Installation Media

Computers that are upgraded from Windows 7 or 8 to 10 have a digital license.

The digital license allows end users to replace drives and clean install without worrying about product keys.

If you had to replace the motherboard then you may need to run the Microsoft activation troubleshooter.

It's always best to have backup options.

You can make a backup image and save the image to another drive or to the cloud.

Then if there are any problems you can restore the image.

These are third party software with free and pay versions:

Acronis, Aoemi, EaseUS, Macrium, Paragon, etc.