I'm using a MacBook Pro (Early 2015) with El Capitan and very often I have to force close LR and repair the drives on which my catalog and photos are stored, because the program stops working and won't let me quit (coloured wheel). It happens often when generating previews. I have GPU turned off.
It never happened with catalog stored on the Mac internal SSD or SD cards. Only external 2.5" physical drives (more than one different model).
Please try optimizing your catalog stored on your external drives.
Also what is the catalog size?
Usually between 50 and 100 MB.
Post the crash report generated after Lightroom crash here.