Oleg,
I believe that is correct that the block not erased is a bad block. In Logic Loader version 2.4.14 you can view the number of bad block using the 'info mem' command.
Here is an example output from a SOM with no bad blocks.
losh> info mem
Configuration of /dev/nand0:
Total Blocks: 4096
Bytes Per Block: 16896
Has Chunks: yes
Bad Block: 0
Total Chunks: 131072
Chunks Per Block: 32
Bytes Per Chunk: 528
Bytes Per Spare: 16
It is not unusual for NAND memory to have bad blocks. Some NAND memory even come with manufacturer-marked bad blocks.
The specification for the NAND used on the i.MX31 allows up to 80 bad blocks. Here is a link to an application note from ST Micro on bad block management.
Here is another forum post on NAND recovery of bad blocks.
Regards,
Brad