1. You back up a Windows machine,
2. Backup finishes with warning "Volume Tracker has failed to freeze checkpoint. A device attached to the system is not functioning"
Volume Tracker is a change block tracking driver that accelerates backup process: changes to the disk or database content are continuously tracked at the block level. This error occurs when Volume Tracker cannot get the necessary data from Microsoft Shadow Copy Provider.
If Volume Tracker was unable to process changes, then you will receive a warning and changes since last backup will be calculated by Agent instead.