I don't know why but Calibre stopped sending working with my Kindle on Ubuntu 12.04 32 bits.

Till a few time ago everything was working just fine, but now whenever I connect my Kindle and try to send some book on the main meory of the device I get this error:

calibre, version 0.8.38

ERROR: Error: Error communicating with device



[Errno 30] Read-only file system: '/media/Kindle/documents/Blisett, Luther'



Traceback (most recent call last):

  File "/usr/lib/calibre/calibre/gui2/device.py", line 81, in run

    self.result = self.func(*self.args, **self.kwargs)

  File "/usr/lib/calibre/calibre/gui2/device.py", line 411, in _upload_books

    metadata=metadata, end_session=False)

  File "/usr/lib/calibre/calibre/devices/usbms/driver.py", line 260, in upload_books

    filepath = self.normalize_path(self.create_upload_path(path, mdata, fname))

  File "/usr/lib/calibre/calibre/devices/usbms/device.py", line 1120, in create_upload_path

    os.makedirs(filedir)

  File "/usr/lib/python2.7/os.py", line 157, in makedirs

    mkdir(name, mode)

OSError: [Errno 30] Read-only file system: '/media/Kindle/documents/Blisett, Luther'

I realize it is something that has to do with the permissions, so I have tried to change them using chown:
sudo chmod a+rwx /media/Kindle
but I get this error: chmod: changing permissions of 'Kindle': Read-only file system

How can I fix this?