Drag and drop won't move ALL selected files between Finder windows
I know this is late but I've found a fix.

First, open disk utility and run first aid on your disk. 

Second, shutdown the computer fully and disconnect all USB devices.

Third, Press the power button and hold command-option-p-r until the Mac reboots again. This will reset the PRAM. 

I hope this fixes your issue like it did for me!