ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
brew update
brew install caskroom/cask/brew-cask
brew cask install osxfuse
brew install homebrew/fuse/ntfs-3g
sudo nvram boot-args="rootless=0"
And reboot.
sudo mv /sbin/mount_ntfs /sbin/mount_ntfs.original
sudo ln -s /usr/local/sbin/mount_ntfs /sbin/mount_ntfs
Based on: http://apple.stackexchange.com/questions/152661/write-to-ntfs-formated-drives-on-yosemite
If your external drive is Seagate, you can use Paragon Driver for free. You can download it form here: http://www.seagate.com/support/downloads/item/ntfs-driver-for-mac-os-master-dl/