Can BTT mount/unmount an external (usb) hard disk?
Is this really the best method?
There is no dedicated action for this (maybe a good addition), however you can easily mount/unmount using the terminal command action.
Basically /usr/sbin/diskutil list will give you a list of all connected drives. Then you need to note the disk identifier, which allows you to mount / unmount disks like this:
Mount
/usr/sbin/diskutil mount /dev/disk1s2
Unmount
/usr/sbin/diskutil unmount /dev/disk1s2
Mm, I don't understand anything about the terminal. If you don't mind, I'd rather wait for a predefined action. I could imagine something like: toggle un/mount (name).
Do you think that would be possible? 
yes that will be possible 
But the terminal stuff is not complicated, don’t be afraid of tying it - you have done much more complicated stuff within BTT
Ah, wait. I just saw that KM can run this as a script. BTT can certainly do that too, no? ... but an action would still be better.
yes that's what I meant, it's called "Execute Terminal Command (Async)" in BTT.
(For more complicated scripts the also available "Execute Shell Script" is better, but you don't need that for this)
Sorry, I didn't realize that. I will try that, thank you!
But you will need to figure out the identifier of your disk, to do this you'll need to use the "diskutil list" command in the actual apple terminal app
//edit: ah actually you can also click the info button in the disk utility app:
In that example the unmount command would be "diskutil unmount /dev/disk3s5"
Understood. That's easy enough even for me Thank you.