The EVS disk and the target ECS must be located in the same AZ.
For a non-shared disk, the EVS disk must be in Available state.
For a shared disk, the target EVS disk must be in In-use or Available state.
The target ECS must be in Running or Stopped state.
A SCSI EVS disk cannot be used as an ECS system disk.
Certain ECSs support SCSI EVS disk attachment. For details, see Which ECSs Can Be Attached with SCSI EVS Disks?

What Are the Requirements for Attaching an EVS Disk to an ECS?