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

[eppur_se_muova]

http://www.macworld.co.uk/news/mac/steps-take-when-your-mac-wont-start-3423817/

This is particularly handy to have if you bought a used Mac and don't have the installer disks for the current system. It's written for 10.8, but some of the tricks go back to the earliest OS X release. I'm going to be trying these out for 10.7, see if I can get my Mac Pro to find the SATA drive again. It can boot from the internal (ATAPI) or USB CD/DVD drive but not from the internal HD I've been using since I bought it -- or from a clone I just made of that drive. Unfortunately, the only bootable CD I can find is Lubuntu -- the Mac Pro 2,1 supposedly can run 10.4 to 10.7, but I can't get it to boot from a 10.6.3 installer disk -- and Lubuntu can't see any SATA disk, only the two CD/DVD drives. I hope it's not the whole SATA bus that's gone bad.

[RoccoR5955]

1. The installer disk that you have

may be in the wrong file format for that Mac.

A mac pro 2,1 seems very old. Perhaps it needs an EFI update.
If you can't see the SATA disk, probably the disk has gone south. Try a different one, just unformat it however you can.

[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.