Not surprisingly, manufacturers typically set the failure level so high that it only triggers when the drive has completely failed and therefore provides little or no warning of impending failure.
I can attest to this behavior from personal experience. Internal hard drive on an iMac was making a funny sound. I ran a S.M.A.R.T check and it came back OK (no details shown in whatever software I used at that time). 48 seconds later the drive died. I was amazed at the lack of warning, especially after a check.
