OS X.5 / X.6 - setting up permissions on external drives:
Setting up permissions on external drives can't be done the "normal" GUI-way. Here's a solution to grant access to an external drive only for specific users. Open the Terminal, be aware of the "." (dot) as the last sign and make sure, that every command is typed in one line.
cd /Volumes/<DriveName>
chmod -N .
chmod +a# 0 "<UserName> allow add_file, search, add_subdirectory, delete_child, file_inherit, directory_inherit, list, read, write, append, execute, delete, chown, writeattr, readattr, writeextattr, readextattr, writesecurity, readsecurity" .
chmod +a# 1 "everyone deny add_file, search, add_subdirectory, delete_child, file_inherit, directory_inherit, list, read, write, append, execute, delete, chown, writeattr, readattr, writeextattr, readextattr, writesecurity, readsecurity" .
chmod -N .
chmod +a# 0 "<UserName> allow add_file, search, add_subdirectory, delete_child, file_inherit, directory_inherit, list, read, write, append, execute, delete, chown, writeattr, readattr, writeextattr, readextattr, writesecurity, readsecurity" .
chmod +a# 1 "everyone deny add_file, search, add_subdirectory, delete_child, file_inherit, directory_inherit, list, read, write, append, execute, delete, chown, writeattr, readattr, writeextattr, readextattr, writesecurity, readsecurity" .
Be aware that the last entry of the ACL has to be the one for "everyone", otherwise all users after this entry wont have access. To see the result use the ls-command:
ls -e