Windows – How to set permissions on external hard drive in Windows 7

No, you will need to take ownership again. The username is irrelevant. Windows uses something called a Secure ID (or SID) to set permissions.

You can use the takeown command in an elevated command prompt that will take ownership of a folder and all its sub-folders, like so (source):

Taking ownership of a folder

Use the following syntax:

takeown /f <foldername> /r /d y

Then to assign the Administrators group Full Control Permissions for the folder, use this syntax:

icacls <foldername> /grant administrators:F /T

The /T parameter is added so that the operation is carried out through all the sub-directories and files within that folder.

Ok, here is the solution I have found. Whether or not this will work with future versions of Dropbox is uncertain. I have opened a service request with Dropbox to try to resolve the problem further.

Overall, the solution is a combination of two things:

1. Setting the umask for the Dropbox process so that newly created files have permissions of 0660. This is user read/write, group read/write, other none.
2. Setting the group for newly created files to the group that needs to have write access to the files in question.

This solution will apply to all files in the Dropbox folder, not just a single file. In my case this is acceptable.

Under Linux, I modify the /etc/init.d/dropbox startup script so that the line invoking dropbox as a daemon reads:

HOME="$HOMEDIR" start-stop-daemon --umask 0006 -b -o -c $dbuser:$dbgrp -S -u $dbuser -x $HOMEDIR/$DAEMON

Adding the --umask 0006 accomplishes setting the umask, and the :$dbgrp portion of the -c option accomplishes setting the group to that the daemon belongs to.

On the Mac side, I run the following commands:

ps aux | grep -i dropbox

From this I can see the command-line options that started Dropbox and from this I extract the $mydropboxid used later. Then I quit Dropbox and open a command prompt and enter the following commands:

umask 0006
/Applications/Dropbox.app/Contents/MacOS/Dropbox -psn_0_$mydropboxid &
exit

I plan to automate the above commands at some point so that I won't have to re-run these any time my machine is rebooted.

This handles setting the mask for newly created files so the group for a file has write access. However, in order to get the group set correctly I need to setgid the Dropbox cache directory - this so far has only needed to be done once:

sudo chgrp -R $dbgrp ~/Dropbox/.dropbox.cache
sudo chmod -R g+s ~/Dropbox/.dropbox.cache

It appears that all new files are first created under the ~/Dropbox/.dropbox.cache directory, so the above commands gives those new files the proper ownership and permissions that new files created by Dropbox has the correct group.