VSFTPD user Home directory

I recently installed VSFTP with IPKG on my Asus WL500gx running 1.9.2.7-7f Oleg's firmware.
VSFTPD is working fine with my admin account an other accounts (I created home directories in /opt/home/userX with specific permissions).
But, with that configuration, I cannot give access to my external USB harddrive (plugged on the router).
The Fat32 partition is in /tmp/harddisk/, I would like to create a "FTP" directory (/tmp/harddisk/FTP) and difine it as the home directory of all FTP users. I did it in the /etc/passwd file.
But, I cannot set any permissions on the /tmp/harddisk/FTP directory, so users cannot logon on the ftp server.
I would like to get one user with read permissions and another one with read/write permissions inside /tmp/harddisk/FTP, how can I do that? What kind of permissions do I have to set up on /tmp/harddisk/FTP?

Thanks for your help, :o

I am considering similar setup for ftp users, separate directories on external usb fat32 disk, one private directory for each user, and some common (pub) directories for more general ftp access. There is however a problem for fat32 partition as it does not support setting access permissions on dirs and files. It may be possibly done using symlinks (or hardlinks?) from users home dirs on ext2 partition to dirs on mounted fat32 and setting access permission on this links, but i am not sure as i am no experienced linux user and not tried it yet myself. I want use this "symlinks method" to grant access for selected ftp users to some shared dirs by creating symlinks from their homes to those dirs too. I dont want my usb disk to be all etx2 formated, as i want use it on windows machine directly too. Can some experienced linux user comment this possible solution?

I also investigated on Symbolic Links, but I think it is not very clean...
I assume it is possible to have a public directory share by several FTP users (with different permissions) on Fat32 as Stupid_FTP works like this!
Please help us!!!

Nobody on this issue?