Most drives I've checked have their "maximum" SMART temp set at 55C. Nothing horrible or magical happens when you hit 55.1C, but at that point you're technically overtemp.
In a room without air conditioning during summer an external without forced air flow could probably get near to or above this figure.
Don't forget it's not just ambient that matters, the heat of the drive itself has to be shed, without a fan it's just by thermal radiation through the shell of the enclosure (which often has a bad thermal connection to the drive itself... in effect it's insulating it!)