Disk Eject
The one and only purpose of Disk Eject is to eject all external disk(s) from macOS.
Why?
There is applescript like this trying to achieve the same goal, but I couldn't get a clean unmount for time machine volume! There were some popup windows showed up during the process saying that some volumes were ejected, that was very annoying. Therefore I decide to roll my own program for this task.
Install
go get github.com/jaux/diskeject
Run
$GOPATH/bin/diskeject
Or, if you have added $GOPATH/bin to your $PATH, you can simply run
diskeject