System protection (System restore) uses a reserved space to backup file versions for every Restore point. Now as this reserved space starts filling up, older versions start getting deleted. Now if you have configured your Windows Backup to save System Image Backups to this drive, then because these image backups take up large disk space, the other file backups (shadow copies) will start getting deleted faster. If this is your case, then Windows warns you about this. So in this case, you can let the reserved disk space settings remain as it is knowing that the shadow copies will get deleted faster or you may increase the Reserved Space allotted to System protection.