Temp ratings are usually listed under product specifications, and you may be able to find some that can withstand high heat.  However the usual ratings for portable external drives appear to be:  Operating Temperature: 5 to 35°C, Non-Operating Temperature: -20 to 65°C.  Converting, 65 C is 150 F, but frankly I'll bet Arizona cars get even hotter.  I, too, melted a pair of sunglasses on a dashboard when there.  If you need to transport one in the summer, I'd use a cooler.