This is checking all the Flash chip and marks the bad blocks and clears the ones that are OK. So often the software marks bads, good blocks, and some bad blocks are still "valid". This is like formatting your Hard Disk, and it does a detailed work to be sure of all the Flash to be 100% reliable. In this case it means that some blocks were bad, but still not marked bad, and were corrupting the image, that wasn't working then properly. A good cleaing is always good to do, even more if you never done it before on a box that has few years age.

WL