OS X 10.11 drops support for many open source protocols used by older NAS devices, specially ones that use the open source software FreeNAS and/or Netatalk. Problems with the following NAS devices brands have been reported, so check for compatibility before upgrading to El Capitan: Buffalo, DataRobotics/Drobo, LaCie, Netgear/Infrant, QNAP, Synology, and Western Digital.