Methods and structure establishing nested Redundant Array of Independent Disks volumes with an expander

Methods and structure are provided for provisioning a Redundant Array of Independent Disks (RAID) volume via an expander that can be used to provision a RAID volume managed by an external RAID controller. The structure includes a Serial Attached SCSI (SAS) expander. The expander comprises physical links with transceivers (PHYs) that directly couple with storage devices, a protocol target and a control unit. The control unit provisions a first RAID volume with multiple storage devices that are directly coupled with the PHYs, and is further masks the storage devices from a SAS domain, by presenting the PHYs directly coupled with the multiple storage devices as a single PHY coupled with a single logical device. The control unit is also operable to provision a portion of a second RAID volume on the logical device in response to the expander receiving a command from a RAID controller.