The LaCie Rugged BOSS SSD device functions similarly to a miniature computer. It uses the metal surfaces of the device to expel heat, so depending on what the device is doing, the top and bottom surfaces can both feel very warm to the touch.
The LaCie Rugged BOSS SSD will typically be the warmest when it is either charging from the wall or powering another device like a portable USB hard drive while doing an ingestion.
There are protections built into the device that prevent it from getting too hot. Once it hits a temperature threshold, the Rugged BOSS SSD will go into a lower performance mode to prevent the device from overheating. If the Rugged BOSS SSD continues to heat up after this point, it will enter a shutdown state and become usable once the battery has returned to an acceptable temperature.
The LaCie Rugged BOSS SSD was designed to operate at an ambient temperature of 0C to 35C. If you use the device in an environment that is warmer than 35C, it may cause the device to reach the temperature thresholds more quickly than normal.