>> [fsck]
> You should then be able to (for each cylinder group):
> - traverse the inode array and remember the blocks that are
>   a) directories
>   b) allocation maps
> - traverse the directory blocks to check the reference counts
> - traverse the alocation maps to find further maps and allocated blocks

Not quite.

Inodes may refer to blocks that are in other cylinder groups.  This
means that you can't know, before having scanned all cgs, which blocks
in the current cg should be allocated, much less which ones belong to
directories.

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