BinaryFruit has driven DriveDx, its Mac OS X solution for SSD and HDD health diagnostics and monitoring, to version 1.4.1. The upgrade supports external USB/FireWire drive S.M.A.R.T. diagnostic on OS X El Capitan.
DriveDx 1.4.1 requires Mac OS X 10.6.8. It can be purchased from the BinaryFruit website (http://binaryfruit.com/drivedx). The Mac App Store update is coming soon. A 10-day trial version is available for download.