How to attach an S3 bucket as a VOLUME to an EC2 instance

You cannot directly attach an S3 bucket as a volume without the use of an additional tool. You can use an S3 Storage Gateway to present a volume, or share from an S3 bucket though: https://aws.amazon.com/storagegateway/

Hi, S3FS may be what you're looking for. Check out https://github.com/s3fs-fuse/s3fs-fuse (if you're under Linux)

I have also used rclone successfully: https://rclone.org/