After about 10 days of obsessive focusing on this issue, I called Apple Support. And guess what? It appears as if they have an answer. Now, I've tried lots of Terminal command lines and rebooting and safe moding and cleaning external hard drives, and I thought that the problem was virtually insolvable. But lo and behold, the support folks at Apple may have given me the answer to all of the frustrations: not being able to transfer large video files from one storage volume to another, whether it's the home volume or not. What the support supervisor did was guided me through a short sequence of actions to repair disk permissions. That seemed to solve the issue. I previously read that, when Apple did away with the disk permissions repair function in Disk Utilities, it was because they incorporated repairing disk permissions during startup. But the support folks told me otherwise, and that every once in a while, disk permissions need to be repaired through the "Get Info" box of the home folder, and a short command line that gets pasted into the Terminal (diskutil resetUserPermissions / `id -u`). That's what I did, and it looks like it's what I needed.
Thank you for your assistance in this matter. You were part of the process that led me to an apparent solution.