No it is not bad sensor. As soon as I unplug external hdd it shows temperature properly. I also tried 3 different ext. hdd's and when plugged in HR21-200 showed 32 degrees. That means that HR21-200 disables internal temp sensor when used with ext hdd. My HR21-700 shows the temp with ext hdd just like yours. It appears that 200 is different in that regard from 700.