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.