You may not need to open the laptop: there is an easier and safer method, by comparing the very first reported temperature value (in Hard Disk Sentinel or your BIOS) with the temperature of the environment (room or office), after a longer power off period (8 hours or so).
Then the sensor in the hard disk should provide almost same value as the temperature of the room. Some Celsius difference (1-2 Celsius) is acceptable - but if the difference is higher (5-8 Celsius or even more, for example on Samsung models and some WD models), setting the Offset in Hard Disk Sentinel may be a good idea to have the real, accurate temperature readings.
For this method (and generally how to set this offset to correct the displayed temperature), please check Help -> Appendix -> Temperature Calibration in Hard Disk Sentinel.
(look for the "Note: if the calibration is not possible (the computer chassis cannot be opened) ...." section at the bottom).
The Help is available on-line also at: http://www.hdsentinel.com/help/en/