Not able to change folder owner rights with chown on Mac OS X Server

Not able to change folder owner rights with chown on Mac OS X Server

I have an external hard drive used for storing and sharing files with
other computers in the network. After a power failure I had issues logging
into the workgroup manager and I noticed that LDAP wasn't running anymore.
I had to recreate the Open Directory Master settings and it seems that all
my previous user data and permission settings were lost.
Now I was able to get into the workgroup manager again and created new
users and put them in a new group ADMINISTRATION. I would now like to give
permissions to this group to read & write files on some folders of the
external drive.
I wanted to start doing that, but noticed that the Admin user (which I use
to login to the Mac OS X server) had no access to the folders on the
external drive. Therefore I thought I had to grant the Admin user read &
writing rights to those folders first.
So I open the Terminal and navigate to the Datadisk and use the command:
sudo chown -R Admin .
This should change the owner rights recursively I thought. When the
command was finished without errors, I checked the Datadisk and strange
enough I now have access to some folders and not to other folders. So I
navigated in the Terminal to one particular folder to test and used the
same command, but nothing changes.
What could be wrong? Hope somebody can help out, because I don't have
access to my data folders anymore so I'm not able to work at the moment.
Thanks in advance!