Add a policy to disable mounting of external storage.

chrome/app/policy/policy_templates.json

Index: chrome/app/policy/policy_templates.json
diff --git a/chrome/app/policy/policy_templates.json b/chrome/app/policy/policy_templates.json
index 1699e208533617a1fb1a9e54f2f7bce22e8c9d54..6450d5b621a2851511548a13a0d18085a3c9d252 100644
--- a/chrome/app/policy/policy_templates.json
+++ b/chrome/app/policy/policy_templates.json
@@ -112,7 +112,7 @@
 #   persistent IDs for all fields (but not for groups!) are needed. These are
 #   specified by the 'id' keys of each policy. NEVER CHANGE EXISTING IDs,
 #   because doing so would break the deployed wire format!
-#   For your editing convenience: highest ID currently used: 150
+#   For your editing convenience: highest ID currently used: 151
 #
 # Placeholders:
 #   The following placeholder strings are automatically substituted:
@@ -2752,6 +2752,23 @@
 
       If this setting is disabled or not configured then users can choose to proceed to the flagged site after being shown the warning.''',
     },
+    {
+      'name': 'ExternalStorageDisabled',
+      'type': 'main',
+      'supported_on': ['chrome_os:0.22-'],
+      'features': {'dynamic_refresh': True},
+      'example_value': True,
+      'id': 151,
+      'caption': '''Disable mounting of external storage''',
+      'desc': '''Disable mounting of external storage.
+
+      When this policy is set to true, external storage will not be available in the file browser.
+
+      This policy affects all types of storage media. For example: USB flash drives, external hard drives, SD and other memory cards, optical storage, punch cards etc.

[Mattias Nissler (ping if slow), 2012/07/11 09:48:13]

I don't think admins will get joke here and some have already been picky about
the description strings in the past, so if I were you, I'd remove the punch
cards here and put them into a code comment if you like to keep them.

[pastarmovj, 2012/07/13 15:06:06]

Done.

+      Internal storage is not affected, therefore files saved in the Download folder can still be accessed. Google Drive is also not affected by this policy.
+
+      If this setting is disabled or not configured then users can use all supported types of external storage on their device.''',
+    },
   ],
   'messages': {
     # Messages that are not associated to any policies.