Dragging the folder to the desktop from the external drive would copy the folder and its contents, not create a alias. If you drag a folder to the Dock, yes, it creates an alias, but only for the Dock. If you drag that alias off the Dock, it disappears in a puff of smoke, so you can't drag it to the desktop, where the OP wants it.
miles01110's response was accurate: Hold the Option and Command keys while dragging the folder from the external drive to the desktop. That will create an alias.