With all the complications HDD manufacturers are constantly trying to make user data storage more reliable. To accomplish that they use various methods and technologies in their drives. 
Figure 5. Control circuit of spindel of HDD (family WDAC 32500 and WDAC 33100)
S.M.A.R.T. (abbreviated Self-Monitoring, Analysis, and Reporting Technology) is intended to inform hard drive users about the status of its main parameters. Many motherboard BIOSes support analysis of those parameters at computer power-up and if some critical parameter exceeds its emergency limit an informational message is displayed during computer start-up. Of course, it does not mean that the drive will stop functioning, but the user should take some steps in that situation, for example, prepare a backup copy of valuable data. If computer BIOS does not contain an analyzer of S.M.A.R.T. attributes you can use an external diagnostic utility launched from within the operating system. The list of such utilities includes, for instance, SMART Vision available from http://www.acelab.ru/products/pc/traning.html.
For greater reliability practically all drives use a technology, which allows hiding and relocation of occurring defects immediately during operation. Some peculiarities of its implementation may vary with different drive models; however, they are all based upon the same principle. If the operating system attempts to access a sector, which cannot be read or written to, then the drive will replace it if possible (if there is sufficient reserved space) with a sector from the reserved zone (assign). The table of thus substituted sectors is stored in drive firmware zone and the drive loads it to controller ROM at power-up.
Impact sensors found in all drives also belong to technologies used for protection against malfunctions. It is a piezoelectric sensor producing an electric pulse at mechanical shock. Filtering of sensor pulses allows identification of obvious impacts. When a drive detects shock action, it parks magnetic heads. One peculiarity of impact sensor installation is the angle of its mounting relative to front case line. It is equal to 45O.
In recent models manufacturers have began to use widely temperature sensors in PCB and heads’ block. Temperature information is monitored by drive processor and the drive stops operation if the allowed value is exceeded. In some drive models temperature is output as S.M.A.R.T. attribute value and there are programs (usually available from the web pages of HDD manufacturers) which allow viewing it.