Hey there, Mattw1013!
Since you'd be using a new mobo, I suppose you already have a new genuine Windows version for it.
If you'd be using this old Windows 7 HDD for the new OS again, you can simply begin the installation procedure and the wizard will allow you to completely reformat it while installing the new Windows version on the drive.
If you will be installing the new operating system on a different HDD or SSD, make sure you install it first
(WITHOUT having that old Win 7 HDD plugged to the SATA ports). Once you have Windows up and running on the primary drive, plug back the old hard drive and
reformat it through Disk Management for your secondary storage purposes.
Hope this helps. Let me know if you have additional questions!
SuperSoph_WD