Please see https://en.wikipedia.org/wiki/IP_Code for more information about IP64 certification.
Can Elgato Thunderbolt™ Drive+ stand up against liquid and dirt?
Your valuable data is always protected by a rock-solid metal enclosure. No dust particle can reach the inside of Elgato Thunderbolt Drive+, and even splashes of liquid can do no harm to your files. This has earned Elgato Thunderbolt Drive+ IP64 certification – for you, it means a drive that’s not only beautiful, but also trustworthy.