As always, I don't work for them, but I really like the product.
"Synchronize Files and Folders" => https://freefilesync.org
It's really just a copy program - pretty typical and standard. Except it does SUCH a good job.
You can set up sessions that copies when you run them - so update, mirror, things like that, to arbitrarily complex sets of disks and files. I use it extensively for backups to always know my files are backed up. It needs to be kicked off manually, but it just works so well and I like monitoring the steps.
Most especially, I move files around between diff folders (think picture sorting, etc) and it will figure that out and "move" the files versus "always re-copy". So it really speeds everything up.
So really what I use it for is "make sure every file is backed up properly" - and it does it really well - it "never gives up until success" no matter what happens. You can just re-run and make sure it's "all up to date".
It supports Windows, Mac, and I think linux - so same on all platforms. It's donation-ware - and I think there is some more features if you donate - but I don't remember since I donated since the very beginning.
No other tool really does it as well - no Windows copy or anything like that gives me any confidence like this does.