What is a Nandroid Backup?
A Nandroid backup is typically performed through a Custom Recovery such as CWM (ClockworkMod), TWRP (Team Win Recovery Project) or Amon_Ra, although at this time Amon_Ra is not available for the Nexus 7.
Nandroid backups are a vital part of rooting, hacking, modding and flashing your device. Once the device is rooted and a custom recovery flashed you can perform a nandroid backup, this is important because once you have root and you have access to the "/" level of the device, you can hack, mod and tweak the device, if something was to go awry during this and the device won't boot or bootloops, you can restore a nandroid backup which restores your device to the point the backup was created.
A nandroid backup is a carbon copy of the devices NAND, the NAND is the flash memory used to store the ROM images on the device. NAND stands for NOT AND which is a form of Electronic Logic Gate, for more information see here
The nandroid copies several images (format, recovery type dependent) of the relevant partitions of the device typically including boot, cache, data, nandroid, recovery and system. The data copied is only relevant to what makes the device boot, this is why a nandroid's size does not match the size of your devices system and other partitions.
During the backup process an MD5 hash checksum is added, an MD5 Message-Digest Algorithim is a crypographic hash function that creates a 128-bit hash value and is used to check the integrity of the backup using a hexadecimal number 32 characters in length.How to create and restore a nandroid back?
First you need to reboot the device into recovery mode whether CWM or TWRP and this can be done a number of ways, either from an app like ROM manager or Goo Manager or if you have an updated device in the case of the Nexus 7 and are running Android 4.1.2 you can cold boot to recovery. To do this, with the device turned off, hold the volume down and the power button until the device boots into the bootloader, now use the volume up and down keys to navigate to "Recovery mode" and the power button the make the selection, you will see the Google splash screen and then your choice of custom recovery will appear.
Once in the custom recovery of your choice, you will either navigate the recovery by touch or by using the volume rocker and power button. TWRP is a touch based recovery and although there is a touch version of CWM avaialble a non-touch version is the usual version of CWM recovery most users will receive.Creating a CWM nandroid backup.
Once booted into CWM Recovery, use the volume rocker to select "backup and restore". Press the power button to select this option and then select "backup", again use the power button to select this option and the custom recovery will perform the backup.
During the process you will see information of the process displayed on the screen including the version of CWM Recovery, SD Card space free (Internal memory) and what parts of the nandroid are being processed. You will also see an area below the Android that shows a graphic of the process time to give you an indication of the time remaining, the time the process takes depends on the size of the ROM and the number of applications installed on your device.
Once complete, navigate to "+++++Go Back+++++", pressing the power key to select and then to "reboot system now" and select to reboot.
If you want to manage backups to regain some of your valuable storage, you will be fine to manually delete backups in the clockworkmod folder using a file explorer but do not to delete any files in the blobs folder as these are shared files files between multiple backups and deleting them will render your nandroid backups useless. The blobs folder can seem large but it is managed by CWM recovery itself so once you delete a backup in the backup folder of clockworkmod folder the next time you run the backup option in CWM will clean up any unneeded blob files.Creating a TWRP nandroid backup.
Once booted into TRWP recovery, tap "Backup" and you will come to the backup option screen, here you have several options. Backup device options, here you can choose which partitions of the device you would like to backup including System, Data, Boot, Recovery and Cache. By default the partitions selected are System, Data and Boot. Recovery backup isn't really necessary on the Nexus 7, the option is your to make. Cache is optional too and it's user choice whether included in the nandroid backup. Obviously although not a huge difference, not including these does make the backup smaller in size.
The next option is to name the nandroid, selecting this will take you to a text input box and keyboard for you to name the backup, by default this will be the date the nandroid is created. If you are naming the nandroid manually after entering the name press Enter on the keyboard to complete the naming process.
You can also enable compression to make the nandroid smaller, ideal for the N7 and it's limited storage and you can also skip MD5 check sum generation.
Once you have made any alterations you would like to make you can swipe the "Swipe to Back up" slider to start the backup process.
As with CWM and although different, relavent information to the backup is displayed in a command style box on the screen including the number of partitions, size of all data in backup, available space etc and you will also see a bar depicting graphically the time remaining until the backup is finished. Once complete a "Reboot System" option will be available onscreen.Restoring a CWM nandroid backup.
Boot the device into CWM recovery and use the volume rocker to navigate to "backup and restore", use the power key to select the option and then select "restore". Select the backup you want to restore and press the power key to start the restore process.
The restore will commence much like the backup process with relevant information being displayed onscreen during the process. Once complete, reboot the device as above.Restoring a TWRP nandroid backup.
Boot the device into TWRP recovery and tap "Restore", select the backup you want to restore and a similar option screen will appear to the backup screen, here you can select which partitions of the backup you'd like to restore, rename the backup and enable/disable the MD5 verification of backups. once you have made your choices which usually would be left as is, swipe the "Swipe to Restore" slider to run the restore.
The restore will commence much like the backup process with relevant information being displayed in the command style window, once complete select to reboot.To be continued...
Please, if you think anything is missing here or incorrect, contact me by PM and I will amend the relevant post giving you credit below that post. I have done this to the best of my knowledge but I as most Android users, I am still learning.
Thanks and if you have question, ask away...
Titanium backups to come...