Disable internal Trackpad

I am pretty sure the trackpad is controlled by an extension(kext), you have to find out which one and remove it from the Extension Folder.
Sadly, I don't have an intel machine running an up to date OS.

What I can say though that even in My extension folder there is one called AppleUSBMultitouch.kext and that one seems to be for an external touch device.
So, to find out how to disable it you have to find out which one controls the internal trackpad.

Edit: There is also a AppleUSBTopCase.kext in there, is that in your extension folder?
Inside that kext is a AppleUSBTrackpad.kext which could also be for an external device, might even control the internal one.
There is also a AppleHIDMouse.kext, maybe that one.

The only way you can find out is to have a second bootup volume in case you mess up.
Lets say you remove the AppleHIDMouse.kext from the extension folder(don't delete, keep in safe place) and restart, if it does not work or can't start up then log into the backup and put the extension back.
Be aware, do this in the root user account, this way it won't change the permissions.

 

I just updated My above post.

Can imagine you don't like it but most of the time it is the only way to find out.
If you have a backup and start from that then you can always go back to the former state without troubles.

Try using this command

sudo kextunload path to kext

This probably won't work if the extension is in use but you can try.

sudo kextload path to kext should enable it again.

And, a restart should always reload the kext so no "permanent" damage done here.

 

Updated/edited above post again.

 

No worries.

You can also check which extensions are running, go into System Profiler, there should be a tab with software and under it is extensions.
On my system (Leopard)AppleUSBTrackpad is running while there is no external connected.
Bit of a shame, I disabled many extensions long time ago and I know from that time there was a couple of times when the trackpad didn't work after removing one, which one I can't remember, I have the AppleADBMouse.kext removed(don't know if it's in your ext. folder) but trackpad still works.

EDIT: I think you are smart enough but just a warning, always boot from your newly backup to see if it works before going ahead with deleting system files.