You can easily verify that the Yahoo content is still there.
In that case, simply start another transfer of the main Yahoo content to the main (parent) groups.io group. Create a new GIO subgroup (with a different temporary name), and transfer the Yahoo subcommittee content to the new GIO subgroup. After you have verified that the new content is complete and correct, delete the current committee subgroup (remember to change its name to a unique random string first), and then rename the temporary subcommittee subgroup to its desired original name.
Should work but don't wait too long.
Art
