I actually had a similar issue. It ended up being a BIOS that refused to detect rc3 is ISO format. Mounting the ISO and copying the files over to a FAT formatted USB got around the boot issue but the qubes installer doesn't see it's root partition then.
The working solution was to insert both the FAT formatted USB and the ISO one (just burned using DD). That way there's an EFI partition for the UEFI to boot and a proper Qubes USB for the installer to use. Not pretty, but it worked.
I didn't try the ISO, and I know little about UEFI booting, but current Linux ISOs seem to be partitioned CD-ROMs: A type 0xef EFI partition (4MB), and a type 0x17 hidden NTFS partition (that is actually ISO9660, containing the actual files)...
Thank you Tobias and other folks and really sorry for delayed reply on this.
This is what I did:-
Updated motherboard BIOS version to latest one.
Verified ISO file should be same as downloaded one, it was corrupted so corrected later.
By doing so, I was able to see Qubes 4.2.0 RC4 installation UI, but was not able to resolve issue of Installation source, so I decided to use Rufus made USB installation media, for this, when I tried to make USB installation media for “Qubes-R4.2.0-rc4-x86_64.iso” by rufus, I was seeing only UEFI (non CSM) option as follows:-See attachment “4.2.0V.png”
but with current stable version of QUBE OS “Qubes-R4.1.2-x86_64.iso”, this issue was not replicable:- See attachment “4.1V.png”
I put more efforts on this and fixed issue at my end itself.
If you see rufus download page “https://rufus.ie/en/”. here a bug has been fixed in Rufus version 4.2, below are fixed issues/enhancements in 4.2 version of Rufus (highlighted one was the issue with me as I was using Rufus 3.x version):-
Version 4.2 (2023.07.26)
Add detection and warning for UEFI revoked bootloaders (including ones revoked through SkuSiPolicy.p7b)
Add ZIP64 support, to extract .zip images that are larger than 4 GB
Add saving and restoring current drive to/from compressed VHDX image
Add saving and restoring current drive to/from compressed FFU (Full Flash Update) image [EXPERIMENTAL]
Fix a crash when trying to open Windows ISOs, with the MinGW compiled x86 32-bit version
Fix an issue where ISOs that contain a boot image with an ‘EFI’ label are not detected as bootable
Increase the ISO → ESP limit for Debian 12 netinst images
Ensure that the main partition size is aligned to the cluster size
so now I have installed Qube OS 4.2.0 RC4 successfully on my machine.
in nut shell, following steps helped me to get fix of this issue:-
Using latest version of Rufus (4.2 or above)
Updating BIOS to its latest version
Verification and usage of corrected Qube OS ISO file