Backup of ECS

Hi all,

I've seen that that this question has already been raised three years ago but didn't find a satisfying answer.

We've stored about one billion documents on ECS via CAS (which is replicated to a second ECS in another datacenter). For legal reasons we are requested to secure these documents outside the ECS on another storage medium (e.g. tape). Do you know if there's any procedure how to manage it?