Step by step guide to partition an external HDD in two file formats

Gparted way

You can use gparted as a powerful tool to accomplish this task. Install it first. Open a terminal (CTRL+ALT+T) and issue the following command

sudo apt-get install gparted

Then open it and follow the pictures-tutorial below.

Attach the external HDD and open gparted. Then select your HDD from the dropdown list.

Unmount the device or else you will not be able to do anything. The "key" must disappear.

Right click on the partition and select Resize/Move

Use your mouse cursor to shrink the partition and leave the space you want. From right to left. From the end to the beginning (always)

After this, right click on Unallocated space and click New.

From the FileSystem drop-down menu select Fat32

Last, click the button in order to apply all pending operations.