[jmesar]

At

- SSH into your Kindle and edit the file /opt/amazon/ebook/security/external.policy
- Add the following 4 lines to the block called 'grant signedBy "Kindlet" {' (this should be somewhere at the top)

Code:
permission java.io.FilePermission "<<ALL FILES>>", "execute";
permission java.io.FilePermission "/tmp/mplayer.liste", "read, write";
permission java.io.FilePermission "/mnt/us/music/-", "read, write, delete";
permission java.io.FilePermission "/mnt/us/music", "read, write, delete";

I got from WinSCP:

Error general (El servidor debe haber facilitado más información)
codigo de error: 4
Mensaje de error desde el servidor: Failure
Codigo de la petición: 3

Solution

With putty
mount root as rw - execute "mntroot rw"

Edit file /opt/amazon/ebook/security/external.policy as indicated in the first post

restore root as ro - execute "mntroot ro"