I can't get beyond 30% of progress bar on booting up Mac Pro, Yosemite o/s up to date. I've done all suggestions re diagnostic and safe boot up etc.

What happens if you boot to Recovery and repair the startup disk?

OS X: About OS X Recovery - Apple Support

30 percent is about as far as the progress bar gets. What is important is what happens next.

If your Mac shuts down, that tells you the drive could not be repaired, the drive could (still) not be mounted, and there is no sense wasting electricity, so it shuts down.

If it does NOT shut down, you could have a graphics card problem that is keeping you from seeing the login screen. Try again in Safe Mode (which does not use the display Driver, it uses "simple" display mode. It also does the disk Repair, and your username and password are required to login and continue.

- What specific Mac Pro do you have? You say "Mac Pro (Pre 2008) running on Mavericks O/S".. Only the 2008 and later Mac Pro desktops can go beyond Lion without a hack.

- You said "

The same, grey screen and 320-30% progress bar."

Does that mean you can't boot to recovery? Are yo using a wired keyboard?

- Try booting from install DVD and repairing the startup disk

Or, try a clean install on another drive and begin from there.

From there you can use Setup Assistant (later).

You could do a checksum on the drive.

"All" depends on who and what. A clean install on a new drive would be next when Safe Boot does not work but the GPU works.

And always have a bootable clone of the system - even a couple so you still have last OS, one from before last set of changes and always have one before you even apply a security update or others.

Even 4GB of memory really is not enough and sounds more like 2008 3,1 using FBDIMMs which today are cheap and easy to find good sets.

And pick up a 250GB SSD while at it!

Clean install is to use the OSX installer to just install OSX. No migration of any data like from Time Machine or installing any apps yet.