I have been bitten by plenty of ticks but to my knowledge none of them had a white spot on their back.
About 2 or 3 years ago my dogs got ticks and I found 2 ticks in my bed but MrD and I were not bitten.
This is an image of the Lone Star tick. I remember seeing a lot of them, especially on sheep when we had to shear them. I used to take a stick pin, heat it with a lighter and stick it into the white spot. Really made them squirm. Those legs were going 100 mph, but it was stuck to the pin.