Would you have been trying to sync your iPhone and suddenly do not believe them backup?
If they have left these text messages:
"Error while trying to copy. There was an unknown error. (-54)"
"Failed to copy because there was an error during backup".
"Error downloading purchased music. There was an unknown error (-50)".
"He has timed out"
"Could not synchronize because it has lost connection"
And they have tried from restarting the device and / or computer, disconnect all other devices connected to the USB ports, uninstalling and reinstalling iTunes, disable firewall, update, configure, disable or even uninstall the antivirus, reinstall the drivers, restore folder lockdown, try a secondary computer and to free up space on the computer ...
I share my solution:
I turned off all Internet connections my computer and uncheck "Find My iPhone" on my iPhone (iOS 8.4 is: Settings / iCloud / Find My iPhone: off). It occurred to me after trying to do everything, even buy another iPhone (lol), and I remembered that in PhoneClean, application I use constantly, always asks me off Find My iPhone, I have no idea why. I do not know if you need to see the internet connection of the computer, but it worked well. Then, iTunes had no mistake and I synced and created the backup in a matter of minutes.
I hope you serve.