The stick is fine. The ISO is fine. But the old PC boots straight into its dead Windows, or shows a boot menu with no USB in it at all. When a GPT stick meets legacy firmware, the machine is not being stubborn — it literally cannot read what you are offering it.
A GPT bootable USB asks the firmware to do UEFI things: find an EFI system partition and run an EFI bootloader. A legacy BIOS — or a UEFI board locked into legacy habits — knows nothing about that. So the stick is skipped silently, which looks exactly like a broken stick. Before you remake anything, understand which of the two fixes applies to your machine: switch the firmware if it genuinely supports it, or remake the stick as MBR, which is the answer far more often than forum posts admit.
👆 Example values from an early UEFI board. The Launch CSM row is the one to hunt for — its name changes by brand.
Legacy BIOS boots by reading a boot sector (that is the MBR world). UEFI boots by reading files from a FAT32 EFI partition (that is the GPT stick you made). Offer a BIOS machine a GPT stick and there is no boot sector it recognises, so it moves on to the next device without a word of explanation. Nothing is corrupt — the two sides simply speak different languages. A truly BIOS-only PC will never boot a GPT stick, no matter how many times you remake it the same way.
CSM (Compatibility Support Module) is a translator built into many UEFI boards: it lets UEFI pretend to be legacy BIOS. Enter setup (usually F2 or Del) and look under Boot for any of these names: Launch CSM, CSM Support, Boot Mode: Legacy/UEFI, or UEFI/Legacy Boot. If one exists, this board can meet your stick halfway. If setup shows no such option anywhere, stop searching — many slim modern boards deleted CSM entirely, and some old boards never had UEFI to begin with. Both cases lead to the same honest answer below.
The cost is real: you are downgrading how that machine boots, for everything, not just this install. If this PC already has a working Windows installed in UEFI mode, flipping to legacy can make that Windows unbootable until you flip back. On a machine with a healthy existing install, do not take this route casually.
No firmware settings touched, nothing on the old PC changed, and the same stick will still boot most UEFI machines that allow a legacy or CSM entry. For a one-off install on an old PC, this is almost always the better trade — change the two-minute stick, not the machine.
The video shows the MBR and GPT options being chosen live and explains this exact old-PC situation — worth three minutes before your next install on an ageing machine.
▶️ Watch Full VideoThat is the known side effect: Windows installed in UEFI mode expects UEFI booting. Go back into setup, set the boot mode back to UEFI (CSM off), and Windows returns. Your install session is the time to use CSM — not a permanent switch on a machine whose Windows was installed the other way.
A black screen after selecting the stick is a different fault from the stick being absent — usually a graphics or very slow USB read issue, not the partition scheme. Try another USB port (a rear USB 2.0 port on desktops), wait two full minutes, and test the same stick on a known UEFI PC to separate stick problems from PC problems.
For genuinely old or unknown machines, yes — MBR is the safe default visit stick. Keep a second GPT stick for modern machines. Two labelled sticks end this entire category of problem, which is what most working technicians quietly do.