OS X stopped auto mounting external drivers My MacBook stopped auto-mounting external hard drives this week.  I have to mount them in Disk Utility, which works fine.  

This has happened with three different drives (which all had Time Machine volumes). Flash drives still auto-mount, as did an SSD.  

I think this may or may not have coincided with installing Parallels Desktop, but I haven't been running it, even in the background.

This is a 2016 USB-C MacBook Pro running 10.14.4.
Encrypted Core Storage and macOS 10.14.4

macOS 10.14.4 changed the behaviour of auto-mounting Core Storage/HFS+ formatted encrypted drives. These drives appear to no longer be automatically mounted.

Please let Apple know this change of behaviour in a minor macOS update is causing you problems. The more people who provide feedback, the better and the better the chance of it being assigned engineering time.

See Automatically Mount Encrypted Core Storage Volume (10.14.4+).