'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 devicesList of devices attached4df7f9574edecf7f        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.abNow 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.