macOS Sierra 10.12.3.
Apple’s macOS just keeps getting more and more unreliable, and for basics—Apple Core Rot.
I was just trying to copy about 900 files from one volume to another (both SSDs). The first couple of tries, the Finder hung when most of the way done, as shown. Copying those same files in Terminal (cp src/*.html dest) works instantly with no issues, proving there is no drive problem.
I gave the Finder several chances: first I force-quit it—same problem. Then I rebooted—same problem. In each case, 'cp' at the command line copied all the files in under a second without any issues whatsoever. The rest of the system is totally stable.
This is not the first time I’ve seen Finder file-copy problems—I’ve seen worse issues with silent failures and files not copied—very dangerous stuff. If I had to guess, I’d say some nitwit got his or her threading code wrong and never bothered to write any proper tests.