[Nexus 7 (2013) Original Android Development] Tasssadar:
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Introduction

MultiROM is one-of-a-kind multi-boot mod for Nexus 7. It can boot any Android ROM as well as other systems like Ubuntu Touch, once they are ported to that device. Besides booting from device's internal memory, MultiROM can boot from USB drive connected to the device via OTG cable. The main part of MultiROM is a boot manager, which appears every time your device starts and lets you choose ROM to boot. You can see how it looks on the left image below and in gallery. ROMs are installed and managed via modified TWRP recovery. You can use standard ZIP files to install secondary Android ROMs and MultiROM even has its own installer system, which can be used to ship other Linux-based systems.

Features:

* Multiboot any number of Android ROMs

* Restore nandroid backup as secondary ROM

* Boot from USB drive attached via OTG cable

You can also watch a video which shows it in action.​

Warning!

It _is_ dangerous. This whole thing is basically one giant hack - none of these systems are made with multibooting in mind. It is no longer messing with data partition or boot sector, but it is possible that something goes wrong and you will have to flash factory images again. Make backups. Always.​

Installation

1. Via MultiROM Manager app

This is the easiest way to install everything MultiROM needs. Install the app and select MultiROM and recovery on the Install/Update card. If the Status card says Kernel: doesn't have kexec-hardboot patch! in red letters, you have to install also patched kernel - either select one on the Install/Update card or get some 3rd-party kernel here on XDA. You are chosing kernel for your primary ROM, not any of your (future) secondary ROMs, so select the version accordingly.

Press "Install" on the Install/Update card to start the installation.
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2.Manual installation

Firstly, there are videos on youtube. If you want, just search for "MultiROM installation" on youtube and watch those, big thanks to all who made them. There is also an awesome article on Linux Journal.

MultiROM has 3 parts you need to install:

MultiROM (multirom-YYYYMMDD-vXX-flo.zip) - download the ZIP file from second post and flash it in recovery.

Modified recovery (TWRP_multirom_flo_YYYYMMDD.img) - download the IMG file from second post and use fastboot or Flashify app to flash it.

Patched kernel - You can use either one of the stock ones in second post or third-party kernels which include the patch, you can see list in the second post. Download the ZIP file and flash it in recovery.

You current rom will not be erased by the installation.

Download links are in the second post.

MultiROM is compatible with both flo and deb.​

Adding ROMs

1. Android

Go to recovery, select Advanced -> MultiROM -> Add ROM. Select the ROM's zip file and confirm. As for the space, clean installation of stock 4.2 after first boot (with dalvik cache generated and connected to google account) takes 676mb of space.​

2. FirefoxOS

Download FirefoxOS ZIP package from this thread. The installation is completely the same as Android ROMs.​

3. Ubuntu Touch

Use the MultiROM Manager app to install Ubuntu Touch.

Ubuntu Touch is in development - MultiROM will have to be updated to keep up with future changes in Ubuntu, so there's a good chance this method stops working after a while and I'll have to fix it.​

Using USB drive

During installation, recovery lets you select install location. Plug in the USB drive, wait a while and press "refresh" so that it shows partitions on the USB drive. You just select the location (extX, NTFS and FAT32 partitions are supported) and proceed with the installation.

If you wanna use other than default FAT32 partition, just format it in PC. If you don't know how/don't know where to find out how, you probably should not try installing MultiROM.

If you are installing to NTFS or FAT32 partition, recovery asks you to set image size for all the partitions - this cannot be easilly changed afterward, so choose carefully. FAT32 is limited to maximum of 4095MB per image - it is limitation of the filesystem, I can do nothing about that.

Installation to USB drives takes a bit longer, because the flash drive is (usually) slower and it needs to create the images, so installation of Ubuntu to 4Gb image on my pretty fast USB drive takes about 20 minutes.

Enumerating USB drive can take a while in MultiROM menu, so when you press the "USB" button in MultiROM, wait a while (max. 30-45s) until it searches the USB drive. It does it by itself, no need to press something, just wait.​

Updating/changing ROMs

1. Primary ROM (Internal)

Flash ROM's ZIP file as usual, do factory reset if needed (it won't erase secondary ROMs)

Go to Advanced -> MultiROM in recovery and do Inject curr. boot sector.

2. Secondary Android ROMs

If you want to change the ROM, delete it and add new one. To update ROM, follow these steps:

Go to Advanced -> MultiROM -> List ROMs and select the ROM you want to update.

Select "Flash ZIP" and flash ROM's ZIP file.

Source code

MultiROM - https://github.com/Tasssadar/multirom/tree/master (branch master)

Modified TWRP - https://github.com/Tasssadar/Team-Win-Recovery-Project (branch master)

Kernel w/ kexec-hardboot patch - https://github.com/Tasssadar/android_kernel_google_msm​

Crowdfunding campaign

A crowdfunding campaign to get test devices took place from July 28th to August 27th 2013. Thanks all contributors, the campaign was successful, raising a total of $562. This enabled a purchase of flo and resulted in MultiROM being ported to said device and the code became much more portable.

These people pledged $10 or more and got perk "The Thread" or "The Code":

flash5000

Bibi

jbaumert

viper08

marius15

x.nicow.x

Ben Hagen

magedragon25

JonathanWardRogers

theperson88

Sean Smith

sabinblitz

If you supported this campaign, either by pledging money or promoting it, I thank you, doesn't matter if you're on the list or not. This wouldn't be possible without you.​

Donations

I'd be glad if you could spare a few bucks. You can use either paypal or Bitcoins, my address is 172RccLB2ffSnJyYwjYbUD3Nx4QX3R8Ris

Thanks to all donors, it is much appreciated :)​