Xperia S Backup & Restore: Could not back up media files and downloaded apps with data When I tried backing up my Xperia S, the default Backup &amp; Restore would backup only system apps.

It says "Could not back up" for "Media files" and for "Downloaded apps with data".

Screenshot:
You do not need to back up media files or app data. They are in device's internal storage. If  you are performing a factory reset these datas wont be erased unless you select Erase Internal Storage when performing a factory reset.

If you still need to backup the media files or app data, connect your Xperia S to a PC and backup the files in the PC.