s3_access_role

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:*"
      ],
      "Resource": [
        "arn:aws:s3:::*"
      ]
    }
  ]
}

sanitised_pry_session

=> true
[2] pry(main)> s3_dev = AWS::S3.new(
[2] pry(main)*   :access_key_id => "destination_access_key",
[2] pry(main)*   :secret_access_key => "destination_secret_access_key",
[2] pry(main)*   :region => "eu-west-1"
[2] pry(main)* )
=> <AWS::S3>
[3] pry(main)> s3_cur = AWS::S3.new(
[3] pry(main)*   :access_key_id => "source_access_key",
[3] pry(main)*   :secret_access_key => "source_secret_access_key",
[3] pry(main)*   :region => "eu-west-1"
[3] pry(main)* )
=> <AWS::S3>
[4] pry(main)> src_file = s3_cur.buckets['some-bucket'].objects['key']
=> <AWS::S3::S3Object:some-bucket/key>
[5] pry(main)> dst_file = s3_dev.buckets['dev-some-bucket'].objects['key']
=> <AWS::S3::S3Object:dev-some-bucket/key>
[8] pry(main)> dst_file.copy_from(src_file)
AWS::S3::Errors::AccessDenied: Access Denied

user_IAM_policy

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "sts:AssumeRole",
      "Resource": "arn:aws:iam::account_number_owning_above_role:role/s3_access_role"
    }
  ]
}