'adb backup -apk -shared -all -f "D:\backup.ab' initiates backup, completes, but produces a file that is only 1kb

I'm attempting to backup my old Android device, a Samsung Galaxy S3, so that I can lend it to my friend while his is away at repairs, then restore it when he's done with it. I have the Samsung USB drivers installed, and the device is recognised. C:\Users\...\AppData\Local\Android\sdk\platform-tools>adb devices List of devices attached 4df7f9574edecf7f device I set a developer password, as per this post , which caused the "Backup starting..." to last about 3 seconds longer than before. It completes successfully, then produces a file that is 1kb. C:\Users\...\AppData\Local\Android\sdk\platform-tools>adb backup -all -f D:\backup.ab Now unlock your device and confirm the backup operation. C:\Users\...\AppData\Local\Android\sdk\platform-tools> Does anybody know what's going on? Thanks in advance.