Maybe you can contact TP-LINK support team and ask them to improve it.

I reported this support ticket:
Hello, I am using the USB sharing feature of this router to share an USB pendrive via FTP.
In the USB pendrive an IP camera will upload snapshots. The camera has an embedded ftp client where I configured de ftp server (192.168.1.1, the router) and user: camera /password: camera. The volume published is "/Snapshots". Of course I granted the access to the volume to that user. After the camera login and attempts to upload the pictures it gets a writing error (no permission) because the directory where it login by default is not inside the /Snapshot, but in an upper directory where it does not have rights to write.
Since the camera can not make a "cd ./Snapshot", of course it is not a human, the upload fails.
Is there any way to configure the home directory for a ftp user ?, for example in /etc/passwd..this will point the ftp user to their home (the volume where the usb stick is mounted) and will fix the problem.
May be a firmware upgrade, where the WebUI show a check box in order to allow the admin set the volume published, as "home directory".
Thanks