title: Error when mounting a read-only persistent disk on Google compute engine
body: <p>I'm having an issue when mounting a read-only persistent disk to a running instance in Google Compute engine.
  The error is:</p>
  
  <pre><code>mount: wrong fs type, bad option, bad superblock on /dev/sdb,
  safe_format_and_mount:        missing codepage or helper program, or other error
  safe_format_and_mount:        In some cases useful info is found in syslog - try
  safe_format_and_mount:        dmesg | tail  or so
  </code></pre>
  
  <p>If I unattached the disk, and re-attach it as read-write, it mounts perfectly fine. 
  So it seems specific to read only disks and I can't seem to find anything in the documentation about mounting read-only disks.
  Any ideas?</p>