Changled submodule cannot be "git added"

And another interesting behaviour of git with submodules:

-------------------------------------------------------------------------------- % git status # On branch master # Your branch is ahead of 'origin/master' by 8 commits. # # Changed but not updated: # (use "git add <file>..." to update what will be committed) # (use "git checkout -- <file>..." to discard changes in working directory) # (commit or discard the untracked or modified content in submodules) # # modified: modules/tftpd_hpa (modified content) # # Untracked files: # (use "git add <file>..." to include in what will be committed) # # modules/ethz_systems_private/ no changes added to commit (use "git add" and/or "git commit -a") % git add modules/tftpd_hpa # On branch master # Your branch is ahead of 'origin/master' by 8 commits. # # Changed but not updated: # (use "git add <file>..." to update what will be committed) # (use "git checkout -- <file>..." to discard changes in working directory) # (commit or discard the untracked or modified content in submodules) # # modified: modules/tftpd_hpa (modified content) # # Untracked files: # (use "git add <file>..." to include in what will be committed) # # modules/ethz_systems_private/ no changes added to commit (use "git add" and/or "git commit -a") --------------------------------------------------------------------------------

- Why can modules/tftpd_hpa not be added?
- Why does git checkout not remove modules/ethz_systems_private/,
  when switching from the master to the private branch?
  (the module is only registered in the private branch)

Nico