How to use condition in the single resource

Terraform
1

Hi Experts,

I would like to create two Instance with different ami’s(Redhat, Ubuntu) and create two volumes and attach to two Instance.
"/dev/sdb" parameter is not supported for Ubuntu when I tried to attach, So in this code, I have created a separate “aws_volume_attachment” resource to attach.

How to use single resource “aws_volume_attachment” using condition if it is redhat use “/dev/sdb” parameter, if it is ubuntu use “/dev/sdg” parameter. Kindly advise

Variable file:

variable "EC2tag" {
  default = ["redhat", "ubuntu"]
}

variable "ebstag" {
  default = ["redhat", "ubuntu"]
}

variable "Algorithm" {
  default = "RSA"
}

variable "redhatdevice" {
  default = "/dev/sdb"
}

variable "ubuntudevice" {
  default = "/dev/sdg"
}

variable "availability_zone" {
  default = "us-east-1e"
}

variable "keyname" {
  default = "test"
}

variable "input" {
  default = {
    ami           = ["ami-0b0af3577fe5e3532", "ami-09e67e426f25ce0d7"]
    instance_type = ["t2.micro", "t2.small"]
  }
}

Main file:

#Creating ec2 resource
resource "aws_instance" "ec2" {
  count             = length(var.ebstag)
  ami               = element(var.input["ami"], count.index)
  instance_type     = element(var.input["instance_type"], count.index)
  availability_zone = var.availability_zone
  key_name          = aws_key_pair.keys.key_name
  tags = {
    Name = element(var.EC2tag, count.index)
  }
}

resource "aws_ebs_volume" "ebs" {
  count             = length(var.ebstag)
  availability_zone = var.availability_zone
  size              = 10

  tags = {
    Name = element(var.ebstag, count.index)
  }
}

resource "aws_volume_attachment" "redhat" {
  device_name = var.redhatdevice
  volume_id   = element(aws_ebs_volume.ebs.*.id, 0)
  instance_id = element(aws_instance.ec2.*.id, 0)
}

resource "aws_volume_attachment" "ubuntu" {
  device_name = var.ubuntudevice
  volume_id   = element(aws_ebs_volume.ebs.*.id, 1)
  instance_id = element(aws_instance.ec2.*.id, 1)
}

#Generate key
resource "tls_private_key" "key" {
  algorithm = var.Algorithm
}

#Attach key
resource "aws_key_pair" "keys" {
  key_name   = var.keyname
  public_key = tls_private_key.key.public_key_openssh
  tags = {
    Name = var.keyname
  }
  provisioner "local-exec" {
    command = "echo '${tls_private_key.key.private_key_pem}' > ./myKey.pem"
  }
}