Ubuntu – Can’t boot after a dd, BIOS reports no bootable drives

I bought a new computer, removed the Windows disk (as usual) and installed 2 brand new 3TB drives. Installed 11.10 from USB stick, apt-get updates and favorite installs, everything was fine. Then I booted from USB stick again and did:

dd if=/dev/sda of=/dev/sdb bs=1M

and after that I cannot boot. BIOS says I have no bootable drives. Don't even get as far as grub. Booting from USB stick sees both drives and the files successfully copied (so I did not dd the wrong direction). It is as if when the second drive was blank the first drive worked, but now that they are identical the BIOS does not know what to do.

Update 1:
Checking the "BIOS confused" idea I physically disconnected one drive and booted from the other. No joy, on either drive. It is as if the blank drive had something that was helping the boot sequence, and the dd overwrote it!

Update 2:
Since it is not even getting to GRUB I thought I would try to recreate the MBR. update-grub just recreated an (identical) grub.cfg. grub-install /dev/sda said "grub-setup: warn: This GPT partition label has no BIOS Boot Partition; embedding won't be possible!" I don't think I want to embed anything, just rewrite the MBR. grub-setup /dev/sda said Segmentation fault (core dumped)

Update 3:
Gave up, did:

dd if=/dev/zero of=/dev/sda

and reinstalled Ubuntu from CD. Still won't boot! It is as if the brand new 3TB drives (Seagate, if it matters) had some little helper boot code on them and when I installed Ubuntu it overwrote that. Of course it is too late to confirm that now, since both drives were overwritten once I did the dd.

Has anyone heard of a brand new drive not being really blank?

Update 4:
OK now my hypothesis is that my BIOS cannot boot to a GPT partitioned drive, at least not a 3TB one. If I repartition it as MSDOS it boots, if I go back to GPT it doesn't again.

Happy to post details but don't know what would be relevant. TIA!