After you have created an EVS disk, you can attach it to an ECS as the ECS's data disk. When an ECS is created, its system disk is automatically added. You do not need to manually attach it.
A data disk can be created when an ECS is created. In this case, you do not need to manually attach it. However, if you separately create an EVS disk, you need to attach it to an ECS.
After disabling a customer master key (CMK) or scheduling the deletion of a CMK takes effect, if you detach an EVS disk encrypted using this CMK from an ECS, the disk cannot be attached to the ECS again because the CMK cannot be obtained. In that case, the EVS disk becomes unavailable.