Hi Norman –
1) I have found the Seagate 1.5 TB drives to be VERY unreliable.
2) I am not a fan of RAID 10. In my opinion, you should take out the four 1.5 TB drives, and stick them in an external box, and run them with a SATA card. I have found that RAID 1 mirroring is a pain in the behind, and that striping two RAID 1 groups to get a RAID 10 is no better off than taking the risk of a RAID 0 with two bare drives. And with the Seagate 1.5 TB drives, in my opinion, you WILL have failures.
I like the Sonnet E4P as a SATA host card, that can run these drives in an external enclosure (not RAID protected).
Just my opinion, and my experience with this stuff.
Bob Zelin