After complete installation and configuration, the root-filesystem usually occupies the whole disk (execpt of course the small boot-partition).
To reclaim space for additional system-partitions you first have to shrink the existing system-partition. This cannot be done while the system is active, so you have to shutdown the system first.
Th shrink the partition you need a second (Linux) system. The recommended
way is to install the desktop version of Raspbian (Pixel). After installation,
you should additionally install the package gparted using the following
commands from a terminal window:
sudo apt-get update
sudo apt-get -y install gparted
To start the program, use the desktop-menu or run sudo gparted from
a terminal.
Gparted will display it's main window. Using the dropdown on the right (above the main pane), select the correct device. Be careful to choose the correct one, if unsure, you can use the menu to query the attributes of the device.
.
As you can see in the screenshot, the second partition occupies almost all of the space of the device. The key symbol shows that the partition is mounted (due to the automounter of Raspbian). Before changing the partition size, you have to unmount the partition (available from the partition menu of gparted).
After unmounting the partition, you can change the partition size again using the partition menu:
.
In the next dialog, you can use the mouse to change the size, or you can enter the values directly:
.
After changing the size of the second partition, you can also use gparted to add additional partitions for multiple system-partitions:
.
In this screenshot you can see tow additional partitions. The third partition will be used as a second system-partition and the fourth partition as a data partition.
To commit all changes, press the button with the green check mark.