The single bay My Cloud units (currently) do not have a fan. Cooling is entirely passive through vent holes, top, bottom and back on the enclosure.
Currently, if one’s My Cloud is using a WD Red hard drive as most if not all first gen V4.x My Clouds do, then the WD Red specifications (http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-800002.pdf) indicate the hard drive operating temperature is 0C to 65C (On the base casting), and non-operating temperature from -40C to 70C.
The WD Blue and Green drives have somewhat similar temperature specs (0C to 60C/-40C to 70C)
