I believe this is the case: The device is divided in a set of block
groups. Every block group comes with his copy of the superblock, all
the bitmaps it needs, a bunch of inodes to use all by itself and its
data blocks. So, this means that we can just use all of the inodes
that are part of the new groups in the new disk as soon as the disk is
plugged in.
As a first implementation we could put the restriction that the new
file system being added shared some properties with the original file
system: block size, blocks per group and inodes per group. As soon as
I finish reading the ext2 source I will comment more on the options we
have here and how many restrictions we need to make.
Cheers,
Miguel.