Can you please tell me what do you mean "bugs" and "issue"?
I see no bugs/issues at all, so wonder why you wrote that...
> On what basis the drive will operate for 700+ days (viability), since after repairing the unstable sectors it has returned to 1000 days again?
When the health of the drive improved, the problems repaired, it is completely normal and expected that the estimated remaining lifetime increases of course.
> Does the HDS application not cause the appearance of further unstable sectors (near existing ones) when trying to analyze the disk or repair.
Generally NOT the software which causes "appearance of further unstable sectors".
If the sectors are not stable (for example due to insufficient power, cable/connection issues) then it is completely normal and expected that new and new unstable sectors detected from time to time. This is independent from software: any read operation can reveal such problems - which may otherwise remain hidden, until the affected sector accessed.
So you may got new unstable sector even during copying a file, opening a document, playing music/video etc.
The read tests of any software (including Hard Disk Sentinel) helps exactly to reveal such possible issues, reveal such weak sectors and generally the problem with the storage subsystem.
This is why it is important to FIX these kind of problems first (by checking connections, replacing cables, fix power issues) - and then performing the disk repair. Otherwise yes, even if you repair problems, new and new unstable sectors will be detected from time to time. The surface test can just reveal them sooner - but it does NOT mean that a surface test function causes the unstable sectors.
> I know it is safe to erase entire partitions or drive without a problem, but the analysis can cause more damage.
If new and new problems detected, then the entire disk surface may be affected, for example recorded in unstable situation. It is very common (for example) when a 2.5" USB hard disk used with single USB 2.0 connection which does not give enough power for proper operation. Then things may work for some time - but the recorded data may not be read back.
On such drives, the best would be to perform a backup and use Disk menu -> Surface test -> Reinitialise disk surface, which would make the drive as "fresh". Further surface tests should not reveal further unstable sectors.
This is exactly described in Support -> Frequently Asked Questions -> What is a weak sector? How to repair weak sectors?
> Is HDS really safe? I do not know how HDS will behave in this situation?
Yes of course. It designed exactly to reveal possible problems which may remain unnoticed - until you'd really need to open a file which can't be read.
> How to configure HDS to immediately inform:
> 1. Sound signal - if there are warnings (eg bad blocks, unstable sectors)
Select Configuration -> Alerts page. Enable the option "sound alert" in the line of "When a new log entry is added".
It means you'll receive sound alert when any new problem detected and logged (in the Log page of the disk drive).
On this page, you can specify the sound alert file and set to repeat sound alert - until you acknowledge by the speaker button in the main window or right click on the tray icon.
> 2. Immediate display of information on the desktop or in the speech bubble in the tray
Select Configuration -> Alerts page (see above) and enable the "Send/display message".
On the Message settings page, you can configure what exactly to be done:
- send email
- display message
- send network message
- add to Windows event log
- run external application
Also you can configure other options, for example to make alert when the health drops below threshold, when temperature is high, shut down PC, etc..
> I do not want to open the application all the time and check that everything is fine with the hard drive.
You do not need if you configured the alerts of course, this is exactly the purpose of those options