title: How can I attach an existing EBS volume when requesting a spot instance on AWS?
body: <p>I am using boto, the code like this:</p>
  
  <pre><code>dev_xvdb = boto.ec2.blockdevicemapping.EBSBlockDeviceType(volume_id='vol-xxxxxx')
  bdm = boto.ec2.blockdevicemapping.BlockDeviceMapping()
  bdm['/dev/xvdb'] = dev_xvdb
  
  rs = ec2.request_spot_instances(price=MY_MAX_PRICE,
                                  image_id='ami-xxxxx',
                                  count=1,
                                  type='one-time',
                                  key_name='MY_KEY_NAME',
                                  security_groups=['default'],
                                  instance_type='t1.micro',
                                  block_device_map=bdm)
  </code></pre>
  
  <p>This code can be run properly, but can't attach the EBS volume(id=vol-xxxxxx). Why?</p>