Goodo, but I don't actually perform file or folder comparisons, as they are unnecessary in my circumstance, and just needlessly shorten the life of your drives. Unfortunately, the situation with HDDs is a Catch-22, and no real guarantees.

I have a download folder for the GOG Downloader, and only new stuff is in that folder until backed up to my other drives, after which it is empty again. Generally I download and relocate within 24 hours. This is also much much faster than running some copy & compare tool ... if you have a lot to compare that is. I'm not overwriting anything, except the odd changelog file.

I just use TeraCopy to make sure the folders and files copy properly in the first place.

I started using TeraCopy, after I found out the hard way with Windows XP, that Windows did not always report failures ... TeraCopy always does, and I have been using it a long time now, sometimes with my companion TeraCopy Timer program.

E.g. DotEmu (and Desura?) have already taught us that a WW3 is not necessarily needed for a digital store to stop its operations.

Didn't Microsoft also have some e-book store for which they suddenly just decided "Nah, we don't feel like doing this anymore... *poof*"?
More advanced filesystems like OpenZFS and BtrFS have an option also for data dedup, I think.

https://en.wikipedia.org/wiki/Data_deduplication

But that has some caveats (like IIRC they require lots of RAM, and in theory could dedup data that is not really identical) and is merely slightly helping the situation. I'd really love to see new technologies offering 1000 terabytes of storage (archival) space cheaply, but now everything seems to be about SSDs where even 1 TB is considered HUUUUUUUUUUUUGE.

"Did you build your e-book library through the Microsoft Store? You'll need to make a change in plans. Microsoft has not only removed books from its online portal, but plans to remove access to existing purchases and rentals in early July."
https://www.engadget.com/2019/04/02/microsoft-store-removes-e-books/

Now add on Games For Windows Live and Zune Marketplace, and Microsoft has already shown us that corporations don't have go bankrupt to take down their digital stores yet remain in business.

MS is one of those "too big/nearly too big to fail" exceptions....many use it so it's hard for many to drop.

Ironically ... perhaps, considering this topic and my earlier replies, I have had to commission a new 4TB HDD, because my main 4TB HDD has become too full.

I have many many HDDs, but I have one main one by Seagate, that has a very handy built-in 2 port USB hub, so it resides beside my PC, but only usually plugged in when I need to use the Hub, or backup something or download something (GOG games) to it, or access something on it, like WIndows or Browser updates, Ebooks, Downloads, Videos, etc.

Anyway, my games from GOG alone are now approaching the 2TB mark, and because that drive also has a lot of other stuff on it that must remain there, it has become time to move my Games off onto another drive. This is especially so, as they are the bigger additions that forever keep requiring more space. So this means I will half fill up my new drive, but also have plenty left over for expansion, while freeing up my main drive for its other stuff, which grows at a much slower pace.

I also gain the option of making my main drive the GOG games overflow drive, as one of my other GOG backup drives is only 2 TB and fast approaching being full. That particular drive though, is split into two partitions, and is the only (MBR) GOG drive my older Win XP 32 Bit PCs can access due to the newer improved storage standard (GPT) for large drives, that XP cannot use unless 64 Bit. So alas I need to come up with a solution for the overflow that Win XP cannot access.

For the time being though, relocate is the name of the game ... and slow but steady.

P.S. My Win XP PCs while great for many games, are no longer given access to the web or anything connected live to the web ... for all the logical reasons.

P.S.S. I forgot to mention earlier, that while the majority of HDDs I now have are USB 3.0, only one of my Laptops has that ability, all the rest of my PCs are still stuck on USB 2.0. I don't use that laptop for chores or regular stuff, so USB 2.0 is my transfer speed.