俺です。

resource aws_instance で作成したEC2へAttachするEBSを作りたい場合、

EBSはEC2と同じAZに作る必要があります。

どうするのが楽かというと、 aws_instance リソースのプロパティを指定するとよいです。

相変わらずGHEの俺俺リポジトリから抜粋ですが、こんな感じになります。

EC2作成

resource "aws_instance" "prd-manage-1a" {
  ami = "${lookup(var.manage_ami, var.region)}"
  instance_type = "${lookup(var.manage_instance_type,"prd")}"
  key_name = "mykey-${var.region}"
  iam_instance_profile = "${lookup(var.manage_iam_instance_profile, "prd")}"
  source_dest_check = true
  ebs_optimized = "${lookup(var.manage_ebs_optimized, "prd")}"
  availability_zone = "${var.region}a"
  security_groups = ["${aws_security_group.prd-manage-1a.name}","${aws_security_group.common.name}"]
  root_block_device {
      volume_type = "gp2"
      volume_size = 64
  }
  count = "${lookup(var.manage_ec2_count, "prd")}"
  tags {
    Name  = "prd-manage${count.index + 1}-${var.az_1}"
    Role = "manage"
    Backup-Generation = "2"
    Environment = "prd"
    Service = "manage-demo"
    Attach_elb = "${lookup(var.manage_elb, "prd")}"
  }
}

EBS作成

resource "aws_ebs_volume" "prd-manage-1a" {
  availability_zone = "${aws_instance.prd-manage-1a.availability_zone}"
  type = "${lookup(var.manage_extend_ebs_volume_type,"prd")}"
  size = "${lookup(var.manage_extend_ebs_volume_size,"prd")}"
  encrypted = "${lookup(var.manage_extend_ebs_volume_encrypted,"prd")}"
  count = "${lookup(var.manage_extend_ebs_count, "prd")}"
  tags {
    Name = "${lookup(var.manage_tag_Name, "prd")}"
    Environment = "prd"
    Role = "manage"
  }
}

後は必要に応じてattachリソース書けばよいですね。