Mint Cinnamon 64bit wont install

[k_zeon]

Hi. I have created a Ventoy USB and  have formatted the ISO partion as NTFS
I have put Mint 20 Cinamon on it. 64bit.
I have an old PC that has Legacy & UEFI.
I can boot to Mint in Legacy and it runs but not in UEFI
I have tried the normal install and screen freezes.
I have tried comatibility but it gets so far then stops at
ACPI: Video Device [VGA] (multi head:yes Rom:no Post:no)

Also just wiped a 16gb usb and installed Ventoy 21 and did nothing else.
copied linuxmint-20-cinnamon-64bit.iso to partition.
Set as UEFI boot mode and still refuses to load either normal or compatability
any help?

Further investigation and I have had Bios set as CSM = Disabled and Boot UEFI only
This does not work

Set CSM=Enabled and Boot UEFI and it works.

I thought UEFI booting did not require CSM to be enabled??
Pls advise why.

tks