When I press 'u' it says 'remote name ('-' for hovered):
What I should insert there?
I mount my USB drive using Udiskie.
The unmount option is for remote and archive mountpoints. I have updated the help. Generally you should use the same program to mount and unmount. In nnn you can mount on demand using the plugin nmount.
Additional reading: https://github.com/jarun/nnn/wiki/Basic-use-cases#unmount-remotes
Most helpful comment
Additional reading: https://github.com/jarun/nnn/wiki/Basic-use-cases#unmount-remotes