Thanks for your message and question.
Yes, I can confirm that the best way to monitor the status (and be alerted about any problems) is to let Hard Disk Sentinel running constantly in the background. As all other software (like a firewall, antivirus software, etc.), yes, it may use some CPU and RAM of the system.
However, during the development of the software it is always a requirement to keep resource usage as low as possible. This allows Hard Disk Sentinel to run perfectly (without noticeable resource usage) on even older computers, like Pentium III systems with Windows NT4/2000 OSes (as many companies still use such systems for production - and for them, it is even more important to monitor the status of the system to prevent data loss).
Usually it may take between 10-15 Mbytes of memory and 0 % CPU, but it may depend on actual configuration and features enabled - as some options (mentioned in the Help) may cause a bit higher CPU usage.
Of course if you prefer, it is possible to start it on demand (for example by using the Portable version): I can confirm that if you completely close Hard Disk Sentinel by File -> Exit, there are no "hidden" resource-consuming background activities (no services, drivers or something similar) remain active.