The current trigger for x-content/image-picturecd is too general - having a non empty folder named PICTURES. There is no device type check, so internal disks, portable usb devices etc are all labelled incongruously as begin a picture cd (eg by nautilus). See downstream bug - https://bugs.launchpad.net/nautilus/+bug/258936.

Especially on case-insensitive filesystems, its not uncommon to have a directory named Pictures, and this still triggers the match.

The offending line in the mime files:

$ grep -A 39 x-content/image-picturecd /usr/share/mime/packages/freedesktop.org.xml | grep -v xml:lang
  <mime-type type="x-content/image-picturecd">
    <comment>Picture CD</comment>
    <treemagic>
      <treematch type="directory" path="PICTURES" non-empty="true" match-case="true"/>
    </treemagic>
  </mime-type>

Ideally this should be enhanced to also include a device type check, or alternatively just removed.

See resulting effect on gvfs ([drive-hardisk] considered a x-content/image-picturecd):

$ gvfs-mount -li | grep -A 15 'Volume(1)'
  Volume(1): Files
    Type: GProxyVolume (GProxyVolumeMonitorGdu)
    ids:
     uuid: '62b4128d-3edc-4dd5-b0d8-7134845d31d1'
     unix-device: '/dev/sda6'
     label: 'Files'
    themed icons: [drive-harddisk-ata] [drive-harddisk] [drive]
    can_mount=1
    can_eject=0
    should_automount=0
    Mount(0): Files -> file:///media/Files
      Type: GProxyMount (GProxyVolumeMonitorGdu)
      default_location=file:///media/Files
      themed icons: [drive-harddisk-ata] [drive-harddisk] [drive]
      x_content_types: x-content/image-picturecd
      can_unmount=1