output.txt

-/+ module.base.mysql_galera.aws_volume_attachment.mysql_galera_ebs_attachment.0
    device_name:  "/dev/sdh" => "/dev/sdh"
    force_detach: "" => "<computed>"
    instance_id:  "i-xxx" => "${element(aws_instance.mysql_galera.*.id, count.index)}" (forces new resource)
    volume_id:    "vol-xxx" => "${element(aws_ebs_volume.mysql_galera_ebs_volume.*.id, count.index)}" (forces new resource)

vm.tf

resource "aws_instance" "mysql_galera" {
  count = "${var.instances}"
  ami = "${var.ami_id}"
  instance_type = "${var.instance_type}"
  vpc_security_group_ids = ["${var.security_groups}"]
  subnet_id = "${element(var.subnets, count.index)}"
  user_data = "${data.template_cloudinit_config.config.rendered}"
  iam_instance_profile = "${var.iam_instance_profile}"
  ebs_optimized = true
  ....
}

resource "aws_ebs_volume" "mysql_galera_ebs_volume" {
  depends_on = ["aws_instance.mysql_galera"]
  count = "${var.instances}"
  availability_zone = "${element(aws_instance.mysql_galera.*.availability_zone, count.index)}"
  encrypted = "${var.ebs_encrypted}"
  size = "${var.ebs_size}"
  type = "${var.ebs_type}"
}

resource "aws_volume_attachment" "mysql_galera_ebs_attachment" {
  count = "${var.instances}"
  device_name = "/dev/sdh"
  volume_id = "${element(aws_ebs_volume.mysql_galera_ebs_volume.*.id, count.index)}"
  instance_id = "${element(aws_instance.mysql_galera.*.id, count.index)}"
}