How hot the room the drives are operating in, what the form factor of the case is like, how the manufacturer has the hard drives crammed into the case, what you are doing with them (moving files from one drive to another, across the network, running stress testing.
My Lenovo ix2-4 NAS runs regularly around 159ºf-172ºf for the CPU, 125ºf-132ºf for the Motherboard, fan runs around 805 rpm. Very small form factor case, and runs hot, due to the drives in there are old school Platter drives. If they were SSD's, they would run a whole lot cooler.
Post what your room temp is, when using the system. I use the older style Radioshack Indoor/Outdoor Weather forecaster for watching our Living room temps, attic & basement with the sensors. You could use a sensor for the model that Radioshack is selling now, inside the case, or something like this to measure inside and component case temps. http://www.newegg.com/Product/Product.aspx?Item=N82E16811998067
This is the newer model that Logisys sells http://www.logisyscomputer.com/viewsku.asp?SKUID=FP206BK&DID=ACCESSORY
Here is another case monitor that uses sensors http://www.xoxide.com/sunbeam-chormatic-windmill-silver.html
There are more out there, just do some searching. Now of course there are also cases that come with these as a part of the case, and the sensors basically just attach to the component you wish to measure, costs will vary though.