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I have a Samsung Galaxy S III i9300. On Windows 7 there is something called a recovery partition which contains a full compressed image of the operating system from which Windows 7 can be installed. On Android, instead, there is a recovery, which as I understand it is a partition with a full micro-operating system from which can be used to decompress an operating system from an image on the sdcard and make it the current operating system.

What I want to ask is. I want to root my device. For this, I can use Odin 3 and ChainFire. Once the device is rooted will I be able to go back and unroot the device? In order to do so I would like to save the program which reinstalls the android operating system from scratch. Where is this program. I want to back it up in case needed, I don't want to lose it or customize it.

I can see my image on sammobile.com, but the download is slow if you do not pay for the download, and I was unable to download the image.

If I install ClockworkMod as my recovery, I need to root the device first before installing ClockworkMod, so when ClockworkMod overwrites the recovery, the old recovery will be gone and I will only be able to restore the rooted version of the OS. Plus, I will not be able to reinstall the old recovery as once overwritten by ClockworkMod this old/original recovery will be gone. Is this correct? What if I want to back up the unrooted version of the OS?

Thanks.

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First, there is no such thing as a "full copy" of the system stored anywhere on the device, as it is with the Windows recovery partition you describe. Neither is there a "program which reinstalls the android operating system from scratch" stored on the device, so you could back that up (installation is always done via , which takes care of itself).

Second thing is a chicken-and-egg problem: in order to create a partition-image backup, you will need access -- so you cannot do that backup before your device is rooted (and a custom recovery is in place to provide the necessary tools). But then you no longer can backup the original state, as that has been altered by the rooting process (and custom recovery installation). So the only way to have the "real stock state" available is to download a stock image.

However, the rooting process itself is reversible (see the tag). And with a full stock installation archive (including the recovery and everything), you should be able to restore the "pre-rooting" state. Not having a Samsung device, I can however not tell about any flash-counters which would let Sammy know what "dirty work" you did in between...

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If there is no full copy of android installed somewhere on the device, then what does the program run with "adb reboot recovery" do? How does a factory reset take place if there is no full copy of android installed somewhere? –  user2239505 May 2 '13 at 16:35
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@user2239505 - Recovery boots you into the recovery, which is a separate small OS, not Android itself. As for a factory reset, what that does is wipe out the /data partition, which contains non-system apps, as well as all user settings and whatnot. The /system partition isn't modified except by system updates. –  Compro01 May 2 '13 at 16:40
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There are two main partitions in Android (think drive letters like windows). /system and /data. /system houses all your OS files and typically remains read-only and off limits to you. /data is read/write and this is where all programs and settings get saved. When you do a Factory Reset, it wipes the /data partition but your /system partition remains unaffected (since it is read-only, it was never modified). When you root, it adds the root files to the /system partition, so when you factory reset, those files still remain. –  bassmadrigal May 2 '13 at 16:42
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