This is UI side unreadable device support.

chrome/browser/chromeos/extensions/file_browser_event_router.cc

Index: chrome/browser/chromeos/extensions/file_browser_event_router.cc
diff --git a/chrome/browser/chromeos/extensions/file_browser_event_router.cc b/chrome/browser/chromeos/extensions/file_browser_event_router.cc
index 5439278847b67ab540568570f784b5c49d0f6ccf..c4182e574202ba9b7ee1c22227da8e867fbc9fe7 100644
--- a/chrome/browser/chromeos/extensions/file_browser_event_router.cc
+++ b/chrome/browser/chromeos/extensions/file_browser_event_router.cc
@@ -332,8 +332,11 @@ void ExtensionFileBrowserEventRouter::DispatchMountCompletedEvent(
 
   FilePath relative_mount_path;
   bool relative_mount_path_set = false;
-  // If there were no error, add mountPath to the event.
-  if (error_code == chromeos::MOUNT_ERROR_NONE) {
+
+  // If there were no error or some special conditions occured, add mountPath
+  // to the event.
+  if (error_code == chromeos::MOUNT_ERROR_NONE ||
+      !mount_info.special_data.empty()) {
     // Convert mount point path to relative path with the external file system
     // exposed within File API.
     if (FileManagerUtil::ConvertFileToRelativeFileSystemPath(profile_,
@@ -353,6 +356,8 @@ void ExtensionFileBrowserEventRouter::DispatchMountCompletedEvent(
 
   if (relative_mount_path_set &&
       mount_info.mount_type == chromeos::MOUNT_TYPE_DEVICE &&
+      mount_info.special_data != "unreadable_unknown_filesystem" &&
+      mount_info.special_data != "unreadable_unsupported_filesystem" &&
       event == chromeos::MountLibrary::MOUNTING) {
     FileManagerUtil::ShowFullTabUrl(profile_, FilePath(mount_info.mount_path));
   }