Any time a drive encounters a bad block the firmware on the drive automatically remaps that block to an available spare sector on the drive. That happens in the normal course of writing to the drive. The critical factor in a surface scan is the discovery of NEW bad blocks which by definition would be data blocks that have gone bad since they were last written to.
1. And if there are no spares, I assume that OS X spits out an I/O error?
2. A surface scan also discovers bad blocks that have
never been written to, which underscores the wisdom of both running an immediate scan on a new drive as a benchmark for future scans and never zeroing all data (which maps out bad blocks) before running a discovery scan.
Edit: So in the sense of case 2, a "new bad block" would be one that has never been written to but has gone bad since it was last scanned.
