I don't suggest moving your My Documents folder or Ringtone folder to Storage card because the Storage card gets power a little later than the device and also after every wake-up from suspended state. Imagine a situation where you have the Ringtone folder moved to Storage card: The device goes to sleep mode after some idle time, you get a call waking up the device. When the device went into sleep mode, the power to SD card gets off. When the device wakes up, there is a delay until the SD gets power and if your ringtone or anything else that need immediate access is there, then the device thinks you have removed the SD and so switches back to default ringtone or in the case of My Documents, it may create duplicate entries in Registry.
Resolution for your above problem of the folder not being available for Sync: Try editing the registry again and soft-reset. After boot, check if the registry changes remain. Then connect for Sync.
Have large file on Storage card always and to keep your SD card synchronized, you can try our own software
SD Sync.
Regards,
Carty..