Hi Leon,
I've been doing a lot of this recently due to playing computer "musical chairs". Your concern can be well founded. People like yourself worry about making a mistake through inexperience. People like myself are scared of making a mistake through lack of attention and presumption.
I'd suggest 4 simple points:
1. install an instance of Teracopy 3.2x
https://www.codesector.com/downloads
2. Always COPY, never MOVE Files. You want to make sure you have a copy of a file at each end of the equation (and they're good) before you clean up behind yourself.
3. Always copy to an empty location. Don't copy "on top of" existing data. which leads to point #4. This is an overly cautious choice. Later down the track, you may not need to abide by this.
4. Be consistent with your method. Set up 2 instances of "My Computer/Windows Explorer" whatever. Your Origin (FROM) being on the left, and Destination (TO), being on the right. This assumes you natively read from left to right. Do this every time. Select your files to be copied on the left window, select copy, then click in the right window and select paste, etc. Before you start copying, double check the locations in the address bar in each window.
With the above followed, you remove the human error in losing data. Sure, you can have bung disks, and that's another story. The beautiful thing about Teracopy, is that after the operation, it will tell you exactly which files did not copy, and perhaps why. Also, if it can't copy something, it will move on, and not halt the process. Great for those multi terabyte overnight jobs.
I hope this helps. Be excellent to yourself.