MP 1,1-5,1 cMP 3,1 won't boot live linux usb ***SOLVED***

[DCBassman]

So, very happy with the hardware on my cMP 3,1 but ElCap is too limited, and patched Catalina is glitchy. I don't have the knowledge to be poking around too deep in MacOS, and it was always my intention to run Mint on my almost equally new-to-me iMac 27 C2D. That machine recognises and boots a live USB installer without any issue. The Mac Pro can't see it at all.
I'd be grateful for any clues as to what 'm doing wrong, as I'm sure I remember my 2,1 being able to see the drive, even if it couldn't make good use of it...
To be clear: the 3,1 currently has no OS at all. If that's a problem, I can slap the original ElCap drive in and see if that makes any difference. In fact, I'll do that right now...

 

[DCBassman]

OK, plugged the ElCap drive back in. The OS can see drives when plugged in, but the pre-boot sequence cannot. Is this a 'feature' of early cMP?

 

[DCBassman]

Thinking back to when I had the 2,1, there was no issue seeing USB sticks in the boot screen.

 

[DCBassman]

OK, a step forward, sort of...
I changed the USB drive I was using. I'd previously tried an 8GB USB2 stick, and a 120GB SSD, to no effect.
Switched to a different brand USB 3 stick. Bingo, there it is in the boot drive selector. Hit it. A GRUB screen! Marvelous! Hit the first entry.
Just a static cursor in top left corner.
Compatibility mode, same. OEM Install, same.
I just though of one more thing I might try, and that's booting the SSD from a drive bay rather than external USB.
I'll be back...

 

[DCBassman]

And...success! It's installing as I type this on the Linux Mint iMac.
Somethings I noticed while prodding about in the interiors of both machines, via Mint system info. The iMac EFI is:
EFI: Apple v: 215.0.0.0.0
date: 06/14/2019
The Mac Pro EFI looks to be the original dated 29th Feb 2008. Has that ever been updated? And if so, how do I update it? I've retained an El Cap hard drive, so I can get back in that way.
Meanwhile, the install is progressing well, no it's finished! Rebooting...
It's alive!

 

[Dayo]

If you are on MP31.006C.B05, this is the last (only) firmware update the MP31 received.

There was apparently an earlier version short lived version but most units actually came with MP31.006C.B05 as it was released shortly after release (Mar 2008): https://support.apple.com/kb/dl95.

 

[DCBassman]

Thanks for that. I had a trawl through updates, but in all honesty am insufficiently clued-up on all things Apple to be sure of what I'm actually doing! I'll put the El Capitan drive back in and try to get it up to date.

 

[DCBassman]

It is, apparently, up to date.
On more pressing issues, I decided, as reasonably happy with High Sierra on the iMac, to use what I'd learned here to push @dosdude1's HS patcher onto this machine, and the technique worked flawlessly.