Thanks Bluestang!

But I should never have run the NAND cleaner. Speed is great at first, but slows down as it struggles with the marginal blocks which have to be re-flagged.

I was always under the impression that the cleaner would help identify bad blocks, not hide them. I guess if you have a drive with a lot of bad blocks, you may have a hard time destructive-flashing firmware. Which I did incidentally, even though it's a brand-new drive.