Lately I have had an issue where copying large files from OSX El Capitan or Windows 8.1 to a Synology NAS have failed.

Disabling IPv6 on the synology NAS seemed to fix this issue.

To do this, go to the Control Panel on the Synology, and in Network, Network Interfaces, you will find the IPv6 setup. Disable this and give it a try. It seemed to fix my problems.