Storageflex 3945N WSS (Windows Storage Server) editions use Intel Embedded Server RAID Technology 2 ESRT2 to mirror operating systems hard disks or solid state disks.
Such mirrored hard disks and solid state disks reside inside the chassis below the top cover. Once a failed operating system hard disk or solid state disk is replaced, the system will automatically rebuild the RAID 1 mirror.
In the event it does not do so, follow the rebuild disk steps within the Configuration Utility (CU) detailed in section 3.6 on page 3-10 of the manual here.