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A surface scan

1. copies the data in the target blocks
2. writes a standard pattern(s) to that data block,
3. reads the block
4. compares what is read to what was written.
5. If they match it goes to step 7 .
6. Else the drive firmware will tray severe attempts to write and read before mapping the current block to a spare.
7. then it writes the original data to whatever block is mapped to that address

So if there was a previous scan, then every data block has been written to and any new bad blocks discovered will be NEW bad blocks.

Now do you see why a full surface scan is so time consuming?