A: As olli said, we need WAY more detail. To begin with: Is this a SAS or SATA disk? The SMART implementation of the two interfaces is completely different. And what did the disk say? And what did you do to "shred" the data?
B: Disks are strange. It's quite possible that in normal operation, the disk was getting so many read errors that it was worried about running out of spare space. After a complete overwrite (which allows re-cycling previously unwritten areas), and perhaps a reformat, it might have more space available: overwriting a previously bad (unreadable) area can cause it to have known good content again.
C: If a disk acts strange, I would throw it in the trash and replace it. Disks are cheap. Your time and your peace of mind are expensive.