My Mac Pro Quad Early 2009 won't boot (have a Samsung 470 SSD as bootup disk). Dreaded interdiction sign. Tried to boot it then from a HDD clone I had - to no evail. Same thing. In target mode from my MBP, both these disks on the MP are accessible and look OK. I could even boot my MBP from the Samsung SSD in the MP via Firewire 800. Very queer. Thank you for any suggestion you might have.