an entire hdd not being used. Should be able to right click the bottom drive and choose format - follow the wizard and create a drive letter and how big you want it to be (just leave it as iis) - that should add a new drive to file explorer and let you relocate any files that you want to move.
can you share a link of your disk management screen?
Depending where it is, you might be able to merge into C or if its not directly behind C drive on the hdd, you should be able to format it and create another partition to use as storage.
an entire hdd not being used. Should be able to right click the bottom drive and choose format - follow the wizard and create a drive letter and how big you want it to be (just leave it as iis) - that should add a new drive to file explorer and let you relocate any files that you want to move.