Can't write on external HDD after upgrading to OS X El Capitan
You have to install Tuxera NTFS or Paragon NTFS, OS X is not able to write on NTFS.

http://www.tuxera.com

https://www.paragon-software.com/home/ntfs-mac/

Edit :

A free solution : http://sourceforge.net/projects/ntfsfree/