In Odoo V11,
In order to fix the problem, you have to override the method `_file_write` in the `ir.attachment` class as following:

class Attachment(models.Model):
    _inherit = 'ir.attachment'

    @api.model
    def _file_write(self, value, checksum):
        bin_value = base64.b64decode(value)
        fname, full_path = self._get_path(bin_value, checksum)
        if not os.path.exists(full_path):
            try:
                with open(full_path, 'wb') as fp:
                    fp.write(bin_value)

            except IOError:
                _logger.info("_file_write writing %s", full_path, exc_info=True)
                # add fname to checklist, in case the transaction aborts

                # Fixed the place the following line in order to fix the bug
                self._mark_for_gc(fname)
        return fname

Hi,

I've got the same problem: for unknown reason, one of the subdirectory of the filestore suddenly "disappeared"

My question is : CAN WE TRUST ODOO ?!!!?

Thanks