You could report to them that you got the error "Operating System not Found" and after a few re-boots it started up properly.
Don't say that you are happy with it now - or that the system is running OK now.
State that you have been told that it "could" be hard drive related and that "it may re-occur".
Make sure they acknowledge your written communication.
This way, if it happens again towards the end of your warranty or shortly afterwards, you have the initial problem recorded within the warranty period.