You can expand storage capacity by attaching DS4000 storage expansion units for configurations up to 33 TB physical storage capacity. Up to six DS4000 storage expansion units (up to maximum of 112 E-DDMs) can be attached to the DS4700 Express models. Model 72 enables up to three DS4000 storage expansion units (up to 64 E-DDMs). The EXP 4 to 6 Attachment option enables up to six DS4000 expansion units (up to 112 E-DDMs).