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.

I should also note that I encountered an issue with SpaceView. The indicator may display loop devices (your system may have loop devices if for instance you installed some snap packages) in the indicator menu, and you can’t filter them out, along with the “Check your disks” message on the panel (because loop devices have 0% free space). Issue reported HERE.

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:

sudo add-apt-repository ppa:vlijm/spaceview
sudo apt update
sudo apt install spaceview

Alternatively, you can manually download the deb from HERE.

Report any issues you may find @ Launchpad.

Source: SpaceView: Ubuntu File System Usage Indicator ~ Web Upd8: Ubuntu / Linux blog