To remove everything (Windows, Applications, Games, Data) on the old removed drive that you want to keep as storage simply reformat the original drive.
Full format recommended. May take some time depending on the size of the drive.
Be very sure that the drive is correctly targeted for the format and that you do not accidentally reformat the SSD.
And if you are not in any hurry or have immediate need of the "storage" I suggest that you just set aside the original drive for a least a few days.
Just in case.....
[Note: Moving post from Systems to Storage. There may be other ideas and suggestions.]