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Tap on Allow and the problem will be fixed. Now connect your device to PC and you will definitely see the said pop-up. Then tap on OK in the confirmation dialogue box that appears. If that pop-up still doesn’t appear, then remove your device from PC, head over to Developer options and tap on Revoke USB Debugging Authorizations. Disconnect your device and reconnect it to the PC to again bring about this pop-up and this time tap on Allow.
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Update it to the latest version from the Android Developer’s forum and proceed ahead. This error signifies that you are using an older build of Android and Fastboot Tools. So head over to Developer Options and do it right away.
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Until you do so, you wouldn’t be able to execute any ADB Commands using the ADB and Fastboot tools. With that said, you would also have to Enable Developer Options and USB Debugging on your Android device. These types of commands are mostly used to flash IMG or zip files, formatting system partition and other such activities. On the other hand, Fastboot commands only work when your device is in bootloader mode. They could be used to boot your device to recovery/bootloader menu, install APK among other things. The former commands could only be used when your device is switched on.
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Just to make things clear, ADB and Fastboot are entirely different in their purpose. Once done, you could then easily execute any ADB and Fastboot Commands.
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ADB and Fastboot Tools, part of the Android SDK and Platform Tool, are used to create an Android Debug Bridge between your Android device and PC.
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