Cloudwatch Alarm for diskspace shows 'insufficient_data'

There are a bunch of windows ec2 instances running certain legacy applications that write application logs to the default path. Hence, we've attached a secondary volume (D: drive , 200GB for the application and it's logs). I'm now trying to create CW Alarms for diskspace using terraform but although the alarms created for each instance, are stuck in 'insufficient_data' state forever.

The terraform snippet for CW alarm is as follows

data "aws_instances" "this" {
  filter {
    name   = "image-id"
    values = [data.aws_ami.this["windows"].image_id]
  }
}

resource "aws_cloudwatch_metric_alarm" "this" {
  for_each                  = toset(data.aws_instances.this.ids)
  alarm_name                = "Disk-space-${each.value}"
  comparison_operator       = "LessThanOrEqualToThreshold"
  evaluation_periods        = "1"
  metric_name               = "LogicalDisk % Free Space"
  namespace                 = "CWAgent"
  period                    = "180"
  statistic                 = "Average"
  threshold                 = "20"
  alarm_description         = "This metric monitors free space on application drive"
  actions_enabled           = "true"
  alarm_actions             = ["arn:aws:sns:xxxxxxx]
  insufficient_data_actions = []
  #treat_missing_data = "notBreaching"

  dimensions = {
    InstanceId = each.value
    Instance   = "D:"

  }
}

I'm guessing I've got the dimensions wrong. I also tried including path = / and device= xvda in dimesions but it still does not work. Any suggestions please?

After a bit of R&D, apparently, what works for metric_name = "LogicalDisk % Free Space" is:

dimensions = {
    InstanceId = each.value
  }

No other dimensions are accepted. The above evaluates the total logicaldisk free space %age of the windows ec2 instance and triggers actions.

I have an open ticket with AWS support and will update here if they provide a different solution.