Native OTG USB disk support for Nexus 7 (WiFi) with Android 4.2.2

On XDA developers there was a discussion on native OTG USB stick support for the Nexus 7 and somebody provided also a patch which should enable this feature. Unfortunately he mixed up several things and the patch was not working. He was quite on a good way, but he missed some things, so I decided to dig around a bit to see how OTG-USB can be supported.

Since Android 4.2 the situation has become worse because of permissions and security changes and multi user support. The patch I am discribing here is not multi user compatible, i.e. all user will see all files on the external disk. So be warned if you want to use this feature.

First I want to describe a bit the modifications which need to be done to enable native OTG USB

1. platform.xml is to be modified in such a way, that media_rw is allowed to access the external storage
   <permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
      <group gid="sdcard_rw" />
      <group gid="media_rw" />
   </permission>
   
2. vold.fstab is to be modified in such a way, that the usb is mounted automatically when the stick is plugged in
   # Mounts the first usable partition of the specified device
   dev_mount usbdisk /mnt/otg-usb auto /devices/platform/tegra-ehci.0
   
3. init.grouper.rc is to be modified that on startup the mount directory is created
   on init
   ...
      mkdir /mnt/otg-usb 0777 sdcard_rw sdcard_rw
   
4. storage_list.xml in framework-res.apk is to be modified that the external storage is added
   <!-- external USB OTG storage -->
   <storage android:mountPoint="/mnt/otg-usb"
      android:storageDescription="@string/storage_usb"
      android:primary="false"
      android:removable="true" />
   

As you can see on the different files to be modified, several components of the Android system are involved, i.e.

• prepare a new ramdisk.img
• prepare a new framework-res.apk
• patch the platform.xml

I prepared an update.zip which applies the required modifications. The update.zip contains:

• a new boot.img including my kernel (based on android-tegra3-grouper-3.1-jb-mr1.1, but I've added DM9601 and KSM support) and a ramdisk.img with the required modifications for the otg-usb support.
• a patched framework-res.apk
• a modified platform.xml

This update is compatible only with stock Android 4.2.2 (JDQ39) and it will break future OTA updates!
If you want to apply this patch, so be sure you know what you are doing and do it on your own risk. I am not responsible for any bricks, data loss, etc.
Please note: I've tested this only on the Nexus 7 (WiFi) and I am not sure, whether it will work on the GSM-version, too, even if I believe it should work, it's disabled in the updater script at the moment.

Another thing to note: Currently only vFAT partitions are recognized. NTFS principially is supported, but vold is not handling NTFS correctly and displays an empty filesystem / a partition to be formatted.

Update 21.02.2013: I've added a pached vold to the new otg-usb-enable-20130221.zip. Now it's possible to mount also NTFS filesystems, but read only at the moment.