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How to perform a NAND backup
Having a copy of your system NAND is important. If your NAND becomes unusable, your 3DS will not be able to boot. Having a backup or your NAND allows you to restore it through GM9 and should get your 3DS back up and running.
Power off your Nintendo 3DS
Press POWER and START at the same time to boot into GM9
Press HOME and navigate to "Scripts..."
Select "Backup SysNAND" and press A
Press A to continue with the backup
This process can take several minutes
Return to the GM9 menu and press START to exit GM9
Decide if you want to move your NAND backup to your PC's storage or if you want to leave it on the SD card. It is recommended that you put it somewhere safe.