A lot of people are not comfortable while using the Mac touchpad so they prefer to use an external mouse. And while using mouse and keyboard both, it is very annoying that the trackpad is also active and changes the mouse pointer location. The best way to figure out this issue is that you should disable the trackpad when the mouse is connected. All you need is to tweak with a few settings that can turn off the trackpad while using a mouse.
Disabling Trackpad when the Mouse is Connected
You will not find these settings under the mouse or keyboard settings, you will need to go through the Accessibility preference in the System Preferences app.
In the Accessibility Section, find the Mouse and Trackpad settings and choose the open ‘Ignore built-in trackpad when the mouse or wireless trackpad is present’. This option automatically disables the built-in trackpad as soon as an external trackpad or mouse is connected.
Under Accessibility, scroll through the column on the left and look for Mouse & Trackpad. Select it and you will see an option on the right called ‘Ignore built-in trackpad when a mouse or wireless trackpad is present’. Enable it and when you connect either a mouse or an external trackpad, the built-in trackpad on your Macbook will not work.
As soon as you disconnect the mouse, the built-in trackpad automatically starts working.
The same setting is not available for the keyboard. In case you still want the same settings for the keyboard you need to have an app known as Karabiner. It can be done by launching the app and choose the settings from the Devices tab according to the needs.