Windows or any other O/S will not "see" the physical disks, it will see the logical drives presentet to it by the RAID controller (Smart Array).
I haven't worked with the P420, but if they havn't changed a lot, it will autoconfigure 2 (same size) drives as a RAID 1 (mirroring).
In you case, you've got a 300 GB RAID 1 :)
BR
/jag