10 steps to take when your Mac won't start up (MacWorld UK) -- hope one works for me.

[eppur_se_muova]

2. It can't see ANY SATA disk. I can boot an iMac (Core2Duo) from the old System 10.7 HD ...

but the MacPro won't boot from it.

If there's a disk there that's the wrong format, it should still show up as a disk, just unreadable. I have two SATA disks that I can read and write from on the iMac (mounted in a USB box) but neither shows up at all on the Mac Pro. These two machines are about the same age -- neither supports 10.8 (that's where you get into the EFI update), but both should support both 10.6 and 10.7.

Oh, and I *never* see a white-screen "sad Mac" or broken folder -- it goes into a UNIX-like inverse-video text-only screen that says "bootable device not found" unless I have that Lubuntu disk inserted and hold down the C key on boot.

So far, it really seems like a problem affecting the whole internal SATA bus. I do have a couple more things to try (from the article) and may even have an empty partition on a FireWire drive that I can try making into a 10.6.8 boot disk (has to be done on the iMac, since I need Software Update). I'm not giving up yet, but it sure is costing me a lot of time to run through all the permuations and combinations -- especially since the Mac Pro never restarts automatically -- have to force power-down.