I believe the P
ontroller (PERC) will be either a PERC5 or PERC6, which you can enter by pressing Ctrl-R
during POST when the PERC is initializing (you should see it on the screen what to press). Once in the PERC BIOS you can allocate any available physical disk storage to a virtual disk (create a new array) so that it is presented to the OS as available storage. The PERC masks any available physical disk storage from the OS until it is added to a virtual disk in the controller. Dell has tools that allow you to manage it in the OS (see: Repository/OMSA
for more information) as well.
I'm guessing here but it is likely that what you call sda and sdb are physical disks (pd0 and pd1) that are mirrored in the controller bios and presented to the OS as sda. As far as configuring pd2 (sdc) and pd3 (sdd), it is a matter of entering the raid controller during POST and creating a virtual-disk (?mirror?) out of the two unallocated disks and then that new virtual-disk should appear to the OS as sdb for your use.
Dell™ PowerEdge™ 2970 Systems
Dell™ PowerEdge™ Expandable RAID Controller 5/i and 5/E
Dell™ PERC 6/i, PERC 6/E, and CERC 6/i Controllers
iSCSI support should be provided by the iscsi-initiator-utils package.