I have this problem recently while charging my Nokia N9 16GB, when it is plugged on an outlet, a pop up appears to select whether to use as mass storage, sync and connect etc.
But if it is plugged on a PC, nothing happens and the pop-up doesn't appear anymore. It just charge like on a normal outlet. It seems that the functionality if it is plugged on an outlet or the PC was switched. I hope you can help me on this as I can't back up my files and settings.
Whilst plugged in to PC what happens if you go to Settings > Accessories > USB?
You can stil backup on device if you go to Settings > Sync & backup.
Symbian specialist until 31/01/2013 with interests in Maemo & MeeGo but an aversion to Windows!
Would suggest you try a hard reset and reinstall PR1.2 using Nokia Software Updater, worked for an issue I had, bit of a pain but my N9 is 100% again now
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