red dots on NAND map when usb updating

[Mitch Bradley]

The red dots in the NAND display ("scan-nand" or "copy-nand" or its 
equivalent) are bad NAND erase blocks.

An erase block is a 128K chunk that has to be erased as a unit.  The 
erasure process gradually wears out the block (charge accumulates in the 
dielectric and shifts the thresholds to the point where that section of 
silicon is forevermore unusable).

A few bad erase blocks per device from the factory is normal.  Over 
time, so more will accumulate, but hopefully not too quickly, if the 
wear-leveling software is doing its job.

The minimum writeable unit is a 2K page, so each erase block has 64 
pages.  Actually, you can write chunks as small as 512 bytes, but the 
hardware ECC operates over 2K chunks, so it's difficult to use sub-page 
writing.