You can load data into an EFS file system from your EC2 instances or from the on-premises datacenter servers that you are using.
EFS file systems is then mounted on the EC2 instance, hence any data that is accessible to your EC2 instance can also be read and written to the EFS. Now if you want to load data that is not currently stored in the cloud, you have to use the same methods you used to transfer the files to EC2, such as Secure Copy. Amazon EFS file systems can be mounted on any on-premises server also, such that any data which is accessible to an on-premises server can then be read and written to EFS using standard Linux tools simple as that.