TIL. On current versions of MacOS, don't use CMD-C/V to move large files around. Drag and drop.
The copy/paste invokes clipboard semantics and engages Handoff and makes large file(s) in /private/var
And eventually it will cause file system corruption on the drive you are copying to if you bump up against your boot filesystems free space.
I just got it to toast a couple of Sandisk Extreme Pro 4TB SSDs, just copy & pasting between them.
All file systems might eventually become unmountable, but disk recovery can see alll the data.
Really weird bug to figure out. Thought it was my TB ports!