Stupid question

how is the hard disk temperature registered ?

is there a senson on the hdd or is the mobo somehow able to sense it ? ( i ask this cuz atm i have 3 HDD ... a really old maxtor 200 GB a newer ( still old but not as old as the other one ) 250 GB Samsung and a few days old 500 GB WD ... i have the old 200 GB and the new 500 GB inside the case and in the Everest app i can see the temps for the 2 hdd inside the case ... i always kept the samsung out of the case cuz it vibrates slightly and inside the case it makes a horible ( bearly audible ) sound which i cant stand when i try to sleep ( computer is on 24/7 )

so im a bit confused as to how my oldest drive has a temp reading and the one bought more then a year after doesnt ( also the wierd coincidence that the only drives with temp reading are inside the case )

if ur curios its a Gigabyte GA790FXT mobo and a Raidmax Ninja black case ...