There are known issues with El Capitan and the 999mb file transfer failure. These issues have been discussed in numerous prior threads. One can use the forum search feature above to locate and read these earlier threads. For starters see the following two threads.
http://wdstagingsplit.staged-by-discourse.com/t/upload-stops-at-999mb/96492/18
http://wdstagingsplit.staged-by-discourse.com/t/support-os-x-el-capitan/96744
