This is my interpretation, others may differ:
The drive you replaced had a value of 63192 for attribute #5, which is sectors that had actually been remapped at some point. That's a huge number of remapped sectors and may have reached the limit of what the drive had available as spares. There were a further 1536 potentially bad sectors waiting to be remapped or recovered. The drive was toast.
Your current ada1 shows 16 potential problem sectors in attribute #s 197 and 198. These sectors may or many not end up remapped. If they do, but then everything stabilizes, the drive may be fine. However, if #s 197 and 198 continue to rise, and/or #5 starts to rise, the drive is probably going bad.
You can read more on Wikipedia.
And be sure to heed the advice to set up a proper SMART testing schedule with functioning email alerts.
Thank you very much for this great explination. I will keep an eye out on those values. I have to figure out now how to get the freenas to e-mail me if there is a problem!