The AppIndicator displays a list of devices in its menu, and clicking on any of them sets it as default, displaying its free space directly on the panel.
From its preferences, you can assign an alias to each device, select the panel icon color, or set the warning threshold (when the free space reaches the threshold, a warning is displayed).
Other features include an option to show the usage of freshly connected devices as notifications, and an option to start SpaceView on startup.
After changing any settings from the SpaceView preferences, click the "Restart now" button or else they won't be applied until the indicator is restarted.
Install SpaceView in Ubuntu
SpaceView Indicator is available in a PPA, for Ubuntu 16.10, 16.04 and 14.04. To add the PPA and install it, use the following commands:
Alternatively, you can manually download the deb from HERE.
sudo add-apt-repository ppa:vlijm/spaceview sudo apt update sudo apt install spaceview
Report any issues you may find @ Launchpad.