LinuxCNC 2.9 Live image won't boot with MBR

Hello, I've been running LinuxCNC 2.7 for 8 years now and I've decided to upgrade to LinuxCNC 2.9 because it's a 32bit installation, and software support is no longer a thing, browser is barely working, no modern SSH keys support (cannot use github anymore) - so I decided to upgrade to LinuxCNC 2.9 by doing fresh installation.

The problem is that PC won't boot from USB. I used dd to copy ISO to USB - and it boots on my modern desktop, but it won't boot on that old PC.
Is it because ISO contains only UEFI and not MBR ? What is the solution here?

Probably, but how old is that PC?

2009.
I managed to boot - the issue was "Legacy USB Support" option in BIOS - it was turned off.

With this option turned on - USB storage is emulated as Hard Disk Drive - and I was able to boot.

But now I'm testing jitter - and it's waaay to high.
With LinuxCNC 2.7 - max jitter was 25k.
With LinuxCNC 2.9 - max jitter is 3 million.

I installed RTAI kernel - but it cannot boot at all (it says give up on waiting root device). PREEMPT-RT works fine.

Any ideas?

What are PC specs. cpu, number of cores,  RAM. 

MB: P5g41t-m lx
CPU: Intel Core2Duo E7600
Memory: 4GB Ram.

Definitely 2.7.