Finder crashes/closes when copying files to NAS
You're experiencing a Samba limitation on the Mac side.  mv works because it's not using Samba.  You have two options: connect to your NAS using AFP (or something else) instead, or update your Samba configuration.

Check your log /var/log/samba/log.smbd for messages containing strings like "This can happen when using 64 bit lock offsets on 32 bit NFS mounted file systems."  If you have that error message, then edit the file /etc/smb.conf to contain these lines:

; Site-specific parameters can be added below this comment.
[Your NAS NAME]
strict locking = no ; END required configuration.