Thanks to sage advice from helpful folks here, I’ve migrated everything from my old HDD to a Samsung EVO 850 SDD. Samsung’s migration tool worked flawlessly. Stoked about that.

New SSD is now my C: drive and I boot to it. My OS (Win 10) is there along with all my programs and everything else that was on the old HDD. All seems well. Want to use old HDD as just a storage device. Is it now safe to wipe it clean? Do I just go into Disc Management and reformat it?

Thanks!

If you are sure everything has moved over, then formatting the old drive will leave you in the same situation you were in before i.e. you will have everything on one working drive. It might be wise to make and save a drive image in the event that your SSD goes bad so you can restore your system in an emergency. But if you do not need the old mechanical drive as a backup of your windows system, then reformatting it will effectively clean it out for whatever use you plan for it. It might be a good time to do a serious test of the mechanical drive using the manufacturer's test software. A full surface scan using that type of software will generally destroy any data on the drive, which is not an issue if you plan to reformat anyway. If you are going to lose control of the drive (give it away for example), then wiping the data with several overwrites is advisable. A simple format does not obliterate the data, which can be recovered with special software. A quick format does not run any tests on the surface, that might flag bad sectors. A full format does check for bad sectors but does not wipe the drive clean. Either will make the drive look like a new drive to Windows.
 
https://www.lifewire.com/how-to-wipe-a-hard-drive-2624527
 

Thanks for the reply. Got me thinking. I do have a disc image of the old HDD on an external drive, but not of the new SSD. Will get that done.