I'm certain it's not 2 years. Just yesterday it was powered off for 15 minutes. may be I misunderstood what you said.Your HDD is already running non-stop for ca. 2 years! Thats incredible and so much! Because this is your D: drive I wouldn't get now in panic. But you should change your HDD in the next months (my recommendation). The values are not "so" bad.
It means that Windows has marked the sectors and is going to disable them to avoid using them. It's only two sectors, but bad sectors are the beginning of the end. As mentioned by @askalan since it's a backup drive it should be OK for a while. Hard Disk Sentinel will tell you if it starts to go down hill. In the settings there is a setting you can set for notifications. Configurations->Preferences->Alerts->check to see a notification when drive health is below 50%/play a sound.Is there any thing I can do about this problem and what exactly does this [ "They may be remapped anytime in the later use of the disk" ] mean?
Hard Disk Sentinel provide a detailed explanation of weak sectors on their website.Is there any thing I can do about this problem and what exactly does this [ "They may be remapped anytime in the later use of the disk" ] mean?
Because weak sectors may not indicate real failure of the hard disk drive, they are not counted as seriously as real bad sectors or other attributes / issues.
... in most cases the weak sectors do not indicate real problems with the hard disk.