I have a VX420-G2v router to which I have connected a hard drive (via the USB port) and configured it to allow external FTP access to the whole drive.
When I try to connect to the hard using FTP across the Internet, I can login and connect OK and I can see the root name of the hard drive (volume(sda2)).
However, when I try to go one level down in the file hierarchy, it takes an age for the remote FTP server to respond (though it does so eventually).
I include below a listing of these transactions (with me using the Microsoft FTP client):
*** Start ***
ftp> dir
---> PORT 192,168,0,30,195,78
200 PORT command successful. Consider using PASV.
---> LIST
150 Here comes the directory listing.
drwxrwxrwx    1 0        0            4096 Jan 01  2016 volume(sda2)
226 Directory send OK.
ftp: 73 bytes received in 0.00Seconds 24.33Kbytes/sec.
ftp> cd volume(sda2)
---> CWD volume(sda2)
250 Directory successfully changed.
ftp> dir
---> PORT 192,168,0,30,195,83
200 PORT command successful. Consider using PASV.
---> LIST
150 Here comes the directory listing.
drwxrwxrwx    1 0        0               0 Feb 05 05:03 @eaDir
drwxrwxrwx    1 0        0               0 Jan 30 21:38 @tmp
drwxrwxrwx    1 0        0               0 Feb 01 04:23 Media
drwxrwxrwx    1 0        0               0 Jan 01  2016 voicemail
226 Directory send OK.
ftp: 259 bytes received in 421.59Seconds 0.00Kbytes/sec.
ftp>
*** End ***
Seven minutes to transfer a short directory listing seems excessive (and indeed most FTP clients timeout before getting this far).
Can you explain what is going on and how I can fix it?

As an experiment, I stopped sharing the whole drive and just shared the "Media" directory (which is at the top level of the file hierarchy). Repeating the FTP gave the following:
*** Start ***
C:\Users\me>ftp nnn.nnn.nnn.nnn
Connected to nnn.nnn.nnn.nnn
220 Welcome to TP-LINK FTP server
200 Always in UTF8 mode.
User nnn.nnn.nnn.nnn:(none)): <MyUsername>
331 Please specify the password.
Password:
230 Login successful.
ftp> dir
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxrwxrwx    1 0        0               0 Feb 01 04:23 Media(sda2)
226 Directory send OK.
ftp: 72 bytes received in 0.01Seconds 10.29Kbytes/sec.
ftp> cd Media(sda2)
250 Directory successfully changed.
ftp> dir
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxrwxrwx    1 0        0               0 Feb 02 01:12 TV
226 Directory send OK.
ftp: 63 bytes received in 0.01Seconds 10.50Kbytes/sec.
ftp> cd TV
250 Directory successfully changed.
ftp> dir
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxrwxrwx    1 0        0            4096 Jan 30 21:44 TV Current
226 Directory send OK.
ftp: 71 bytes received in 421.56Seconds 0.00Kbytes/sec.
ftp>
*** End ***
Once again, the first directory listing took 0.01 seconds (as did the second) but the third took 421 seconds (almost exactly 7 minutes)! On other occasions, I got similar results but the slow transfer took 210 seconds, exactly half of 7 minutes. I feel this figure of 7 minutes has to be meaningful. Any ideas?