The old videos drive has been swapped out (again) for a new one. The old one has been placed into a USB 3.0 adapter connected to my 'main brain' machine where I've run all sorts of scans and diagnostics and found no problems whatsoever. As for the new one, the original problem still exists but only on one out of four Windows machines I've tested it with, and none of the Linux machines. Of course-- Murphy's Law-- that's the machine I usually want to copy from (the Theater machine). Of note: When that one Windows machine has the copy failure it causes the drive to go offline for everyone, even on the server itself. The problem only occurs on the 'Videos' share, I can copy the same huge files to the other the other three shares without issue on said machine. I have been trying to isolate what is different about that machine and/or that share and so far I'm coming up blank. Every machine of course has different hardware and some of the software varies by machine but nothing that seems even remotely likely to be the culprit. If anything, it should be my main machine with the problem since it is loaded to the hilt with all sorts of software imaginal. I have meticulously dug all through the network settings and everything there is identical except for the IP address. I still suspect some sort of buffer overflow since it only occurs with big files. I will continue to troubleshoot this problem as I get time between other projects because it's nagging me too! The videos share is actually the least important one. I set it up for the enjoyment of all the other household members yet they very rarely use it. I will be sure to post the cause and solution when I figure it out.
Thank you for your help and time!

Can you boost memory so the OS can allocate more I/O buffers?
It may not help, but I have seen situations where it DID.

I would still look for another (probably non-gui) way to transfer the file. #1 I still think that may be the problem, and #2 even if it is not it may work AROUND the problem.

Both machines have plenty of RAM, the Windows machine has 16GB, the server has 48GB ECC so I'm googling trying to find out how I can increase default buffer sizes for TCP and otherwise. So far- have only found info for samba, still looking.
My workaround for now, is to copy to a different share (misc files share) into a folder named "!Videos_MoveUs!" and then remote into the server and move them from that folder to the appropriate location on the videos drive. So far that is working. I'm a little paranoid since I have yet to figure out why the videos share behaves differently, they all have the same parameters in samba, so I'm also stashing backup copies to a removable drive. But you make an interesting point about GUI, I think I may try it from the Windows Command Prompt instead of File Explorer.
That is my goal. I am trying to wean myself off of Windows. I'm getting fed up with Microsoft's recent shenanigans. Problem is I know Windows very well and when something goes wrong I can fix it. Linux, not so much. I also have a ton of licensed Windows software, some it very specialized such as my Christmas light sequencing, and a lot of custom stuff I have written for myself that I really don't think I'll be able to port.

I'm currently running an ffmpeg script to see which videos have become corrupted and which are still OK. With nearly 1000 movies and shorts, and 5000 TV shows totaling 3.5TB I am currently guestimating that script will take 3-4 days to complete based on its progress so far.

I understand the problem. I still have a licensed copy of OS/2 WARP in the original box, and I hate to pitch it. (Good OS! Microsoft should have leveraged that agreement instead of cheating IBM!)

Luckily, most of my old software was in assembler and is no longer needed, or in Pascal and I can port using FreePascal quickly, or rewrite it in V-lang, Rust, or C if I need time and that level of performance.

Let me know if I can help, and there are others here who are very willing to help.