try this from techarena
Error 21 Unknown
Mac, Go to ~ / Library / iTunes /. In the Finder, you can choose Go> Go to Folder. Type or paste the path name above and click OK. - Delete all files in the iPhone Software Updates and Device Support. - Try to restore the iPhone again.
On Windows, these files are located in a hidden folder. To view these files in Windows Explorer, you must select Tools> Folder Options, click View, then select "Show hidden files and folders.
In Windows Vista, follow the instructions below: - Go to Start> Computer> Local Disk and type the following path: C: / users / [yourusername] / AppData / Roaming / Apple Computer / iTunes / - Delete all files in the iPhone Software Updates folder and Device Support. - Try to restore the iPhone again.
In Windows XP, follow the instructions below: - Choose Start> Run, then type C: / documents and settings / [yourusername] / Application Data / Apple Computer / iTunes / - Delete all files in the iPhone Software Updates and in Device Support.
If, despite these recommendations, you still have this error, make sure you have the latest version of iTunes.
Try using a USB hub.