OSX El Capitan / Not Recognizing USB Hard Drive
I have a Seagate external drive and found that the NTFS for Mac OS X had to be removed and reinstalled. (Apple logo / Preferences / NTFS for Mac OS X - right click and remove)  This link will take you to the free update.  It worked for my Seagate drive but no my Octigen one.
http://www.seagate.com/gb/en/support/downloads/item/ntfs-driver-for-mac-os-master-dl/