You need to install special driver to perform diagnostics of external drives.
Mac OS X does not support diagnosing external drives using S.M.A.R.T. technology “out of the box” . In order to allow your Mac to diagnose external drives, you will need to install a special third party driver. Please note that this is a requirement of Mac OS X, and not DriveDx.
The standalone version of DriveDx includes this driver. But users also can download and install (other version of) driver by themselves. See installation instructions bellow .
When I first installed DriveDx I was unable to access my external until I installed the driver...SATSMARTDriver.
