While the Trackpad on a MacBook can perform almost all the functions of a Mouse, there are many users who prefer using the Mouse and want to disable the Trackpad.
Another reason for disabling the Trackpad is to prevent the possibility of the Trackpad registering unwanted clicks, due to accidental brushing of the palm or fingers, while you using a Mouse.
Disable Trackpad on Mac When Mouse is Plugged In
The option to disable Trackpad on Mac while it is attached to mouse is available in the Accessibility settings on your MacBook.
Before going ahead with these steps, make sure that Bluetooth is enabled on your MacBook, so that the external mouse can connect to your MacBook using Bluetooth.
1. Disable Trackpad on Mac When Mouse is Plugged In
You can find below the steps to disable the built-in trackpad on your MacBook, whenever it is connected to a Mouse.
This setting ensures that an alternate input method in the form of Trackpad is available on your Mac, in case you forget to bring the mouse with you.
Note: It is recommended that you follow these steps only if your MacBook is connected to a mouse. This ensures that your Mac has an alternate input method, prior to disabling the Trackpad.
1. Click on Apple Logo in top-menu bar and select System Preferences in the drop-down menu.

2. From the Settings screen, go to Accessibility > Mouse & Trackpad and select Ignore built-in trackpad when mouse or wireless trackpad is present option.

After this, the Trackpad on your MacBook will be Automatically disabled, whenever it detects an external Mouse being plugged-in to it.
When you Unplug the Mouse from MacBook, the Trackpad will be Automatically enabled and you will be able to use it.
Can Disabling Trackpad Cause Any Problems?
The only possible consequence of disabling the Trackpad on a MacBook could be a slight increase in power consumption, while using a wireless mouse.
Apart from this negligible point, we haven’t come across anything else that can be listed as a drawback or possible consequence of disabling the Trackpad on a MacBook.
At any time, you can always go back to using both the Trackpad and Mouse on your MacBook by going to System Settings > Accessibility > Trackpad and disabling “Ignore built-in trackpad when mouse is present” option.