ISO images bootable only in Legacy BIOS OR (!) UEFI ? Decision at ISO creation time or at storage on USB flash drive?

pstein

Extraordinary Member
As you known there are many sofwtare tools available which offer not only a setup.exe but an ISO to download.

This ISO can either be burned on a CD/DVD or put onto an USB Flash drive.

I am NOT talking about Win10 ISO images but other software like (examples):
- Gparted GParted -- Live CD/USB/PXE/HD
- Partition Wizard Bootable Partition Manager| MiniTool Partition Wizard Bootable Edition
- Clonezilla Clonezilla - Downloads

The problem is NOW that when I store one of these ISOs onto an USB Flash drive (with the famous, well known Rufus v3.5 tool) then
they are oftentimes bootable only on Legacy BIOS computers.

On other UEFI-based computer (with disabled Secure Boot) the same USB flash drives are not even recognized in Boot menu.

Why?

Are ISO images only created for Legacy BIOS OR(!) UEFI ?
Or can they be created to work with both BIOS types?

How do I find out for a given ISO if its working with (only) Legacy BIOS or UEFI (without trying it out)?
Is there a tool which shows it or a file inside the ISO which indicates it?

At which point resp. step is the decision made for which BIOS type it is working?
Is it done at ISO creation time or is it done at storage time on USB flahs drive (=inside e.g. Rufus)?

Thank you for commenting
Peter
 

Neemobeer

Windows Forum Team
Staff member
An ISO is nothing more than a container. When an ISO is "burned" to any storage media the contents are extracted from ISO and stored on the media so in short being an ISO has nothing to do with booting.

Things that do matter
  • The partition scheme (MBR vs GPT)
  • The filesystem (NTFS should work but it's recommended to use fat32)
  • The boot files available from the ISO

For UEFI boot (secure boot enabled or disabled) the partition scheme needs to be GPT and for legacy it needs to be MBR
On a GPT disk there is a fake MBR record for compatibility and I believe Rufus creates a real one for what they call a hybrid partition scheme (not officially supported)
 

ragnarok1968

Well-Known Member
An ISO is nothing more than a container. When an ISO is "burned" to any storage media the contents are extracted from ISO and stored on the media so in short being an ISO has nothing to do with booting.

Things that do matter
  • The partition scheme (MBR vs GPT)
  • The filesystem (NTFS should work but it's recommended to use fat32)
  • The boot files available from the ISO

For UEFI boot (secure boot enabled or disabled) the partition scheme needs to be GPT and for legacy it needs to be MBR
On a GPT disk there is a fake MBR record for compatibility and I believe Rufus creates a real one for what they call a hybrid partition scheme (not officially supported)
I concur. The ISO is essentially or actually an image of a CD/DVD. When you install it to say a USB drive, you are creating an installation media of the original. I have come to realize that with Windows10, We use GPT I believe. I'm an old codger now and we used to have to learn to work with the MBR. there were commands back in the day for repairing a bad MBR.

Earlier in the year, I bought a DellG3 laptop since I don't game much anymore, I gave my wife my old Msi laptop. I recall being able to install Windows 7 on it using a work around and could get Win7 installed on my SSD NVMe drive by some slight of hand. I notice then as I type this that, Even with Win7 we were dealing with GPT partitions as opposed to MBR. Don't ask me to explain this, I just can't.

I always ensure, when possible, that my BIOS is set to UEFI with secure boot. Legacy just runs you through the gauntlet of post-testing and other nonsense. Times have sure changed.
 
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