google compute instance with 2 disks: http://stackoverflow.com/questions/42169895/what-is-the-correct-way-to-add-a-second-disk-using-terraform-on-google-compute

main.tf

variable "image_url" {
  default = "debian-cloud/debian-8"
}

resource "google_compute_disk" "seconddisk" {
  name = "seconddisk"
  type = "pd-standard"
  zone = "us-west1-a"
  size = "100"
}

resource "google_compute_instance" "someinstance" {
  name         = "someinstance"
  machine_type = "n1-standard-4"
  zone         = "us-west1-a"

  disk {
    image = "${var.image_url}"
  }

  disk {
    disk = "${google_compute_disk.seconddisk.name}"
  }

  network_interface {
    network       = "default"
    access_config = {}
  }
}

output

$ terraform apply
google_compute_disk.seconddisk: Creating...
  disk_encryption_key_sha256: "" => "<computed>"
  name:                       "" => "seconddisk"
  self_link:                  "" => "<computed>"
  size:                       "" => "100"
  type:                       "" => "pd-standard"
  zone:                       "" => "us-west1-a"
google_compute_disk.seconddisk: Still creating... (10s elapsed)
google_compute_disk.seconddisk: Creation complete
google_compute_instance.someinstance: Creating...
  can_ip_forward:                                      "" => "false"
  create_timeout:                                      "" => "4"
  disk.#:                                              "" => "2"
  disk.0.auto_delete:                                  "" => "true"
  disk.0.disk_encryption_key_sha256:                   "" => "<computed>"
  disk.0.image:                                        "" => "debian-cloud/debian-8"
  disk.1.auto_delete:                                  "" => "true"
  disk.1.disk:                                         "" => "seconddisk"
  disk.1.disk_encryption_key_sha256:                   "" => "<computed>"
  machine_type:                                        "" => "n1-standard-4"
  metadata_fingerprint:                                "" => "<computed>"
  name:                                                "" => "someinstance"
  network_interface.#:                                 "" => "1"
  network_interface.0.access_config.#:                 "" => "1"
  network_interface.0.access_config.0.assigned_nat_ip: "" => "<computed>"
  network_interface.0.address:                         "" => "<computed>"
  network_interface.0.name:                            "" => "<computed>"
  network_interface.0.network:                         "" => "default"
  self_link:                                           "" => "<computed>"
  tags_fingerprint:                                    "" => "<computed>"
  zone:                                                "" => "us-west1-a"
google_compute_instance.someinstance: Still creating... (10s elapsed)
google_compute_instance.someinstance: Creation complete

Apply complete! Resources: 2 added, 0 changed, 0 destroyed.

The state of your infrastructure has been saved to the path
below. This state is required to modify and destroy your
infrastructure, so keep it safe. To inspect the complete state
use the `terraform show` command.

State path: terraform.tfstate

$ gcloud compute instances describe someinstance --zone=us-west1-a --format json | jq '.disks|length'
2