OS X stopped auto mounting external drivers
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+).