System Administration Guide, Volume 1

Bad Block Number Checks

The fsck program checks each block number claimed by an inode to see that its value is higher than that of the first data block and lower than that of the last data block in the file system. If the block number is outside this range, it is considered a bad block number.

Bad block numbers in an inode might be caused by an indirect block not being written to the file system. The fsck program prompts you to clear the inode.