If your application has something regarding its state saved on a disk, you need to remove it before applying Auto Scaling. What was previously saved as files in the EC2 machine must be removed and managed by a service. There are two options which are as follows:
AWS Elastic File System (https://aws.amazon.com/efs/): In a few words, this is a network file system that is mounted in your EC2 machine with virtually infinite space where you only pay for the space used by your file.
AWS S3 (https://aws.amazon.com/s3/): This was the first AWS service on the market and is an object storage designed to deliver 99.999999999% durability.
In general, the S3 should be your favorite solution, but it is not always applicable because it requires application software change to use it. Consequently, in some cases, you may need an alternative that you can leverage on EFS.
The world is full of software and plugins designed around S3. For example, WordPress saves the files...