Comment # 31 on bug 968727 from Franck Bui

(In reply to patrick shanahan from comment #29)
> runlevel 5, systemd-228-10.2, external usb sdb mounted at:
>   /run/media/paka/TOSHIBA EXT
>   /var/run/media/paka/TOSHIBA EXT
> (Don't know why it mounts two places)

It's not mounted twice, /var/run is a symlink to /run.

> 
> switch to runlevel 3
> 
> tty6 previously open is dropped, relogged on
> /dev/sdb is no longer mounted
> 

>From your logs:

Mar 03 12:11:56 toshiba systemd[1]: multi-user.target: Trying to enqueue job
multi-user.target/start/isolate
Mar 03 12:11:56 toshiba systemd[1]: udisks2.service: Installed new job
udisks2.service/stop as 19463
Mar 03 12:11:56 toshiba systemd[1]: udisks2.service: Changed stop-sigterm ->
dead
Mar 03 12:11:56 toshiba systemd[1]: udisks2.service: Changed running ->
stop-sigterm
Mar 03 12:11:56 toshiba systemd[1]: Stopping Disk Manager...

as expected the service that mounted your USB disk was stopped because it's not
part of the multi-user target.

So this seems to work as intended.

Are you sure that tty6 was open before switching from runlevel 5 to 3 ?

Still from the logs:

Mar 03 12:11:56 toshiba systemd[1]: autovt@tty6.service: Failed to load
configuration: File exists
Mar 03 12:11:56 toshiba systemd[1]: autovt@tty6.service: Trying to enqueue job
autovt@tty6.service/start/fail