[Galaxy Tab 2 7" Android Development] Nicene Nerd:
Dual-booting has always intrigued me. I dual-boot between Windows and Linux on one of my computers, dual-boot between stock firmware and RockBox on my Sansa Fuze v2, and I decided to find out how to dual-boot my Galaxy Tab 2 7.0". It took a couple days of work, but I have found one way to do so using the external SD card.

In the future, if I can work it out, I will provide a method for dual-booting using only internal storage. Update: I have successfully tested an internal dualboot setup. I will post details later.

Warning: While I am not aware of any specific way this could brick your device, stuff happens, so don't blame me. I've tested this and found no problems, but you may not be the same.

To use this method, you need a microSD card at least 8GB in size. If you use 8GB, though, you will have pretty limited space for storage, so a card 16GB or greater is advised.

Preparing SD Card

Repartition your microSD card using whatever program suits you. You need to shrink the first storage partition to make room for the new system partitions. The new partitions should take at least 5GB, so be sure to leave at least 5.25GB. You can always resize later if you need to.

After shrinking your storage partition, create four ext4 partitions in this order: system (min 800MB, recommended 1.25GB), cache (min 500MB, recommended 700MB), data (min 3GB, recommended 5GB), and EFS (min 21MB, recommended 32MB).

Verify your partitions on your tablet by inserting the SD card, booting into CWM, and running "parted /dev/mmcblk1" (no quotes) over ADB shell.

Flashing External ROM

Now you just need a ROM to flash. Most ROMs are not packaged to run from external SD card. You MUST either use a provided ROM or edit one yourself. I provide here a repackaged version of the 8/28 CM10 nightly which can flash to and run from your SD card. If you want to edit a ROM yourself, see the bottom of this post.

Boot into CWM and backup. You will need your backed up boot image if anything goes wrong.

Now flash your ROM. It will install to the external SD card on the appropriate partitions if you did everything right. Reboot: you should be in your new ROM.

You'll probably want a Google Apps package. See below for packages for external dualboot.

Using Your Dual-Boot Setup

Your internal and external ROMs do not share internal storage, due to the odd nature of the SD card fuse tool.Your external ROM has an "internal SD card" of its own. You may find a way to mount the internal /data/media to the external ROM's /sdcard. My new ROM conversion script will add attempt to mount the internal ROM's storage to /storage/intSdCard

The ROMs I provide here include a script in /system/bin called "bootinternal." Run this from a terminal emulator or adb to switch from the external ROM back to the internal one. I have attached a script called "bootexternal" to this post. Use it on your internal ROM to switch to the external one (remember to set the executable permission).

In order for the boot switching scripts to work, you must do some preparation. On your external ROM, place the internal ROM's boot.img in /sdcard/Boot/internal.img. For your internal ROM, place your external ROM's boot.img in /sdcard/Boot/external.img.

If you ever need/watch to switch ROMs manually, run the following commands in a terminal emulator or over ADB, replacing "boot.img" with the boot image of the ROM you are switching to:

cat boot.img > /dev/block/mmcblk0p5
reboot

Alternatively, you could switch boot images using Mobile Odin.

Update: I would skip using my prepackaged ROMs now, because I have uploaded a converter script that will actually do a better job. These will configure the "bootinternal" and "bootexternal" scripts automatically.

If you ever need to wipe your data, cache, or dalvik cache, see the next post for CWM zips.

Downloads

Roms for External SD Card:

CyanogenMod 10 Nightly (8/28/2012)

AOKP P3113 Stable

Note that these are probably unnecessary now, because I provide a script to convert ROMs automatically on Linux.

Google Apps Packages for External SD Card:

Gapps for Jelly Bean - Use for CM10, AOKP, etc

Gapps for Ice Cream Sandwich - Use for stock, RomsWell, etc

If I ever have time, I plan to upload SD card images which you can use to partition your external SD card automatically.

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Repacking a ROM for Dual-boot from SD Card

Note: You no longer need to do this manually. I have uploaded a script which automates the conversion process. See the next post for download.

First, you need to extract and unpack the boot image. There are tools and scripts available for this, so find one.

Once the boot image is unpacked, you need to edit init.espresso.rc on the ramdisk. Find where the partitions are mounted and change the code to look like this (yours may look slightly different depending on the ROM, but just be sure to replace the default partition mounts with those of your SD card partitions):

#mount ext4 /dev/block/platform/omap/omap_hsmmc.1/by-name/FACTORYFS /system wait rw
mount ext4 /dev/block/mmcblk1p2 /system wait rw
#mount ext4 /dev/block/platform/omap/omap_hsmmc.1/by-name/FACTORYFS /system ro remount
mount ext4 /dev/block/mmcblk1p2 /system ro remount
#mount ext4 /dev/block/platform/omap/omap_hsmmc.1/by-name/DATAFS /data wait noatime nosuid nodev crypt discard,noauto_da_alloc
mount ext4 /dev/block/platform/omap/omap_hsmmc.1/by-name/DATAFS /data2 wait noatime nosuid nodev crypt discard,noauto_da_alloc
mount ext4 /dev/block/mmcblk1p4 /data wait noatime nosuid nodev crypt discard,noauto_da_alloc
#mount ext4 /dev/block/platform/omap/omap_hsmmc.1/by-name/CACHE /cache wait noatime nosuid nodev nomblk_io_submit,errors=panic
mount ext4 /dev/block/mmcblk1p3 /cache wait noatime nosuid nodev nomblk_io_submit,errors=panic
#mount ext4 /dev/block/platform/omap/omap_hsmmc.1/by-name/EFS /efs wait rw
mount ext4 /dev/block/mmcblk1p5 /efs wait rw

Repack the boot image and stick it back into the ROM. Then edit the updater-script, and change all references to the internal system partition (/dev/block/mmcblk0p9) to the external system partition (e.g. /dev/block/mmcblk1p2). If any other partitions are referenced, change them as well.

Optional: insert the "bootinternal" script in /system/bin and the a script to mount the internal storage in /system/etc/init.d.

Your ROM is ready to go. Flash and be happy!

Thanks to Johnsel for helping me edit the boot image and pointing me to sendust7's work on the Atrix, who I also thank for his precedent.