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To cut off the Network connection, directly pull out the network cable connected to the computer. If you are using Wi-Fi, just disconnect it.
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If your computer stucks while you are trying to take operations to Backup and Sync, and there is no important document data being saved, you can just shut down your machine to stop Google photos backup. If your computer stucks completely and you can't do anything on it even though you have tried manually closing the computer, you have to cut off its power supply by disconnecting the power cable or pull out the battery.
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