Did you create the user for ftp usage?
What is the username? ftp_user?
Do you want the ftp folder to be: /etc/sda1/ftp_folder or /mnt/sda1/ftp_folder?
Or /etc/sda1/ftp_folder/ftp_user or /mnt/sda1/ftp_folder/ftp_user ?
local_root will be put all users in that directory after ftp login.
Better don't use that?
Assuming your ftp username is ftp_user
In /etc/passwd you will find something like
ftp_user:x:1000:1000:GROUP:/mnt/usb:/bin/false
/mnt/usb is the home directory.
Change that to your desired home directory (e.g. where the user is put after (ftp) login)
/bin/false is the path of the users shell. You want to set this to /bin/false
Only opening port 21 on wan side will not work.
To make file transfers work you also have to specify a passive port range and open those ports.
@tmomas
Thanks. I edited my post.