On a CRC error, ETFS can apply error correction coding (ECC) to attempt to recover the data. This is suitable for NAND flash memory, in which single-bit errors may occur during normal usage. An ECC error is a warning signal that the flash block the error occurred in may be getting weak, i.e. losing charge.
ETFS will mark the weak block for a refresh operation, which copies the data to a new flash block and erases the weak block. The erase recharges the flash block.