Possible Bad Disk
I have a Hitachi SATA drive, it's about 3 years old, has not given me any problems. I was able to get a free copy of Ashampoo HDD Control. According to it the drive is 100% healthy, someone recommended a couple of other programs to check the drive, one Acronis free drive monitor. With ADM the drive shows on 18% healthy, with Fail on Reallocated Sector Count and degradation on Reallocation Event Count.
I also tried HDD Sentinel and it showed the drive at 24%.
I used the Hitachi and WD tools to check the drive and both reported no errors
As I said the drive has not given me any problems so far
If Hard Disk Sentinel is showing the disk as degraded as well as ADM, then I'd be wary of that drive.
The problem is that there is no actual SMART standard in as much as what features the various manufacturers enable and the raw data interpretation.
If you want to another sanity check, download the trial version of Disk Keeper (defragmentation software) as the latest version (v12) comes with a SMART reporter as well. So if this also shows the disk as degraded, then I'd be likely to believe the majority concensus.