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To create a bootable USB, download the operating-system image from its official publisher, write it to a correctly identified flash drive with a compatible imaging tool, then start the target computer from the USB entry that matches its firmware mode. “BIOS” may mean legacy BIOS/CSM, while newer computers use UEFI; choosing the wrong mode can prevent Windows Setup from installing correctly.
The safest approach is to plan the firmware mode first, then follow the image publisher’s instructions instead of applying a one-size-fits-all BIOS recipe. A USB can be perfectly written and still fail to boot if you select the wrong entry, use an unsupported filesystem, or change security settings unnecessarily.
What “BIOS bootable USB” actually means
A bootable USB contains startup data in a format the computer’s firmware can load. The firmware may be traditional legacy BIOS, BIOS compatibility mode (CSM), or UEFI. A modern computer can show two entries for one drive, such as UEFI: USB and BIOS: USB. They are different boot paths, not two copies of the drive.
Before creating media, decide how the operating system will be installed. For Windows, Microsoft documents that starting Setup in the wrong firmware mode can produce a partition or installation mismatch. Select the UEFI entry for a UEFI installation and the legacy/BIOS entry only when the computer and installation plan require it. Check the computer manufacturer’s instructions for the supported modes and boot-menu key.
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Use the image publisher’s instructions ahead of generic BIOS advice. Ubuntu’s current guide, updated October 13, 2026, covers Ubuntu-family images and identifies different tools for Ubuntu, Windows, and advanced command-line workflows: Ubuntu’s bootable USB documentation.
What you need before writing the drive
- The official image: Download the ISO or other installation image from the operating-system publisher. Verify that it supports the target computer’s firmware mode.
- A spare USB flash drive: Writing normally formats or overwrites the selected device. Copy anything important elsewhere first.
- Enough capacity: Microsoft’s documented manual Windows flash-drive procedure requires at least 5GB of free space. That is a requirement for that procedure, not a universal minimum for every operating system or image release.
- A computer on which to create the media: Use a writer intended for the image and the operating system running on this computer.
- The target computer’s boot instructions: The key and firmware menu layout vary by manufacturer.
Do not proceed until you can identify the USB by its size, model, or storage label. A mistaken selection can erase another external drive or the computer’s own disk.
How to choose the USB and writer
There is no single capacity or price that applies to every installation image. Match the drive to the image size and the publisher’s instructions. Microsoft specifies at least 5GB free for its documented manual Windows flash-drive procedure; a current Linux image or another Windows release may need a different amount. A spare drive whose contents are already backed up is usually the most practical choice because writing media is destructive.
Compare creation tools on four useful points: whether the tool supports the image, whether it runs on the computer you are using, whether it can produce media for the target firmware mode, and how clearly it identifies the destination before erasing it. A graphical tool that labels the drive by model and capacity is easier to audit than a command-line destination typed from memory.
| Method | Best fit | Firmware and image notes | Main caution |
|---|---|---|---|
| Startup Disk Creator | Ubuntu and Ubuntu-based images created from an Ubuntu desktop | Included with the extended Ubuntu Desktop installation; intended for Ubuntu-family media | Ubuntu warns it may not work correctly with unrelated operating systems |
| Rufus | Writing an ISO from Windows | Can create media for the mode supported by the selected image; GPT plus UEFI (non-CSM) is Ubuntu’s fix for a particular modern-system failure | Review the selected device and partitioning options before starting |
| balenaEtcher | Users who need a multi-platform graphical writer | Ubuntu lists it as an option for creating bootable media | Confirm the destination drive because writing is destructive |
dd |
Experienced users who need a command-line workflow | Writes an image directly to a device | A wrong destination can destroy the running system; identify the device first |
The tool does not replace firmware planning. A correctly written image can still fail to appear if the computer is set to an incompatible mode or if you choose the wrong boot-menu entry.
Step 1: Confirm the target mode and image
- Read the target computer’s specifications or firmware documentation and determine whether it supports legacy BIOS, UEFI, or both.
- Decide whether the installation should use legacy partitioning or UEFI partitioning. For a Windows installation, keep the intended mode consistent from the first boot through Setup.
- Download the image from the operating-system publisher. Do not merely copy the ISO file to the USB; use an image-writing tool or the publisher’s documented procedure.
- If the firmware later shows separate UEFI and BIOS entries for the same drive, choose the entry matching your installation plan.
If you are replacing an existing system, note its current mode before beginning. Changing from legacy to UEFI or the reverse can alter how the installer expects the disk to be partitioned. When the manufacturer documents a special recovery or boot procedure, use that procedure instead of guessing at firmware settings.
Step 2: Prepare and protect the USB drive
Insert the flash drive and close programs that might be using it. Back up its contents. Ubuntu warns that its writer workflows destroy data on the selected device, and Microsoft’s manual Windows process formats the drive.
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Disconnecting unrelated removable drives is a useful safety measure, especially when using a command-line writer. If several drives remain connected, identify the target by its reported model, capacity, and connection before accepting an erase prompt. Never assume that the first device in a list is the USB you just inserted.
Step 3: Write Ubuntu or another Linux image
Startup Disk Creator on Ubuntu
- Open Startup Disk Creator from the Ubuntu applications menu.
- Choose the downloaded Ubuntu image as the source.
- Select the intended USB device as the destination. Check its capacity and name carefully.
- Start the write operation and approve the warning that the destination will be erased.
- Wait for completion, safely eject the drive, and insert it into the target computer.
Startup Disk Creator is designed for Ubuntu images and Ubuntu-based derivatives. Ubuntu cautions that it may not work correctly with unrelated operating systems; use that operating system’s own recommended writer for those images. If the application lists more than one removable device, stop and verify the selection rather than relying on its position in the list.
Rufus on Windows
- Connect the flash drive and open Rufus.
- In Device, select the intended USB drive. Confirm its size so another disk is not overwritten.
- Choose the downloaded image file.
- Review the partitioning scheme and target-system fields against the target firmware.
- Start the write process, accept the erase warning, and wait until Rufus reports completion.
- Eject the drive safely before moving it to the target computer.
Ubuntu specifically advises that if a Rufus-created USB fails on a modern system that disables legacy compatibility, set the partitioning scheme to GPT and the target system to UEFI (non CSM). That is troubleshooting guidance for that situation, not a universal setting for computers that must boot legacy BIOS. If the target really requires legacy boot, use the image’s documented legacy-compatible settings instead of copying this UEFI fix.
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Ubuntu lists balenaEtcher as a graphical option available on multiple platforms. Select the image, select the USB destination, and start the write only after checking the device identity. Follow the current instructions for the image you are writing if the tool presents image-specific options. A successful completion message confirms the write operation, but you still need to select the correct firmware entry on the target.
Advanced dd workflow
Use dd only when you are comfortable identifying disks from the command line. First list storage devices with your operating system’s disk utility, disconnect unrelated removable drives where practical, and record the USB device name. The destination must be the whole USB device, not one of its existing partitions. A wrong destination can overwrite the operating system that is currently running, so a graphical tool is safer for most readers. Ubuntu includes dd as an advanced option in its bootable-media guidance.
Do not improvise a command from a different operating system. Device names and unmounting steps differ, and a typo can target an internal disk. Confirm the device again immediately before executing the write, then wait for the command to finish before removing the USB.
Step 4: Create Windows installation media manually
Microsoft’s documented manual flash-drive procedure uses FAT32 and states that FAT32 can boot both BIOS-based and UEFI-based PCs. The drive needs at least 5GB of free space for that procedure.
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- Mount or open the Windows installation image.
- Copy the installation files to the USB.
- Eject the drive and test it from the target computer’s boot menu.
FAT32 cannot store a single file larger than 4GB. If sourcesinstall.wim exceeds that limit, follow Microsoft’s documented workaround for an oversized file or use a supported media-creation path that handles it. Do not assume that any arbitrarily large Windows image can be copied to FAT32 unchanged. See Microsoft’s Install Windows from a Flash Drive instructions.
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The FAT32 limitation is about the size of one file, not the total capacity of the drive. A drive can have adequate free space and still reject the copy because one installation file crosses the 4GB limit. Check the file named in the error before reformatting or replacing the drive.
Step 5: Boot the target computer from the USB
- Insert the completed USB into the target computer.
- Restart the computer. If it is powered off, turn it on with the USB already inserted.
- Open the one-time boot-device menu or enter firmware setup. Common keys include Esc, Delete, F1, F2, F10, F11, and F12, but the exact key depends on the manufacturer. Ubuntu notes that F12 is often used; consult the model’s manual.
- Select the USB entry for the intended mode: UEFI: USB for UEFI or the corresponding legacy/BIOS entry for legacy boot.
- Confirm the installer or live environment starts. If it does not, shut down and recheck the media and mode before changing unrelated firmware settings.
Microsoft’s mode guidance explains the distinction between UEFI and legacy BIOS boot entries: Boot to UEFI Mode or Legacy BIOS mode. Some systems require a key press immediately after power-on, while others expose a boot menu through a firmware button or an advanced restart screen. The manufacturer’s instructions take precedence over a generic key list.
If the USB appears twice, do not choose randomly. Select the entry that matches the installation mode you selected earlier. Starting Setup through one mode and then changing to the other can recreate partition or boot-loader problems.
Secure Boot: change it only for a specific problem
Secure Boot is a UEFI security feature, not a universal “USB boot” switch. Do not disable it automatically just because you are installing from a USB. If a particular medium is rejected, consult the operating-system and computer manufacturer’s current instructions first.
Microsoft says Secure Boot may need to be disabled temporarily in some troubleshooting cases and recommends enabling it again after the issue is resolved. Follow the manufacturer’s firmware procedure and record the original setting before changing it. Microsoft’s guidance is available at Windows 11 and Secure Boot.
Legacy mode and Secure Boot are separate decisions. Enabling legacy compatibility does not automatically solve a signature problem, and disabling Secure Boot does not make media written for the wrong partition scheme compatible. Change only the setting tied to the documented error, then restore the security setting when instructed.
Troubleshooting by symptom
The USB does not appear in the boot menu
- Confirm the image was written as bootable media rather than copied as one file.
- Recheck the destination device and recreate the USB if necessary.
- Use the manufacturer’s boot-menu procedure and try the other listed USB entry if both UEFI and BIOS entries are present.
- Verify that the image supports the target firmware. For a particular Rufus failure on a modern system without legacy compatibility, Ubuntu recommends GPT with UEFI (non-CSM).
- Try the USB after a full shutdown as well as a restart, because some firmware menus are only available during the initial power-on sequence.
Windows Setup reports a partition or mode mismatch
Restart and select the firmware entry that matches the intended installation. Microsoft documents that Setup can fail when it is started in the wrong mode. Do not switch randomly between entries after partitioning has begun; decide which mode you want and restart Setup consistently in that mode.
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The writer reports that data will be erased
Stop and verify the drive’s model and capacity. If the USB contains needed files, cancel, back them up, and restart the process only when the correct device is selected.
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The Windows image will not fit on FAT32
Check whether an individual file exceeds FAT32’s 4GB limit, especially sourcesinstall.wim. Use Microsoft’s documented workaround for an oversized file or another supported creation method rather than forcing the copy.
Secure Boot blocks the medium
Check the image’s compatibility and the hardware vendor’s instructions. If temporary Secure Boot disablement is the documented fix, re-enable it after the installation or troubleshooting step, as Microsoft recommends.
The computer starts from its internal disk instead
Reopen the one-time boot menu and select the USB explicitly rather than relying on the default boot order. Confirm that the USB is connected before power-on and that you selected the correct UEFI or BIOS entry. If it still skips the drive, recreate the media and verify that the writer completed without an error.
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The installer starts but cannot find the expected disk
Stop before deleting partitions. The issue may be related to the installation mode, firmware storage settings, or the image’s hardware support. Return to the manufacturer’s documentation and the operating-system installer guidance; changing unrelated firmware options can create a second problem.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Final checks before installing
- The image came from the official publisher and is intended for the target operating system.
- The USB’s contents are backed up or disposable.
- The writer completed without errors and the correct physical device was selected.
- You know whether the target should boot UEFI or legacy BIOS.
- You know the manufacturer-specific boot-menu key.
- You will change Secure Boot only when a specific compatibility issue requires it, and you will restore it afterward when advised.
- You know which USB entry to choose if the firmware lists both UEFI and legacy versions.
- You have a recovery plan for important files before changing partitions or reinstalling Windows.
Frequently asked questions
Frequently Asked Questions
Can I just copy an ISO file to the USB?
Usually not. Copying the ISO as an ordinary file does not create the boot structures firmware expects. Use the operating system’s recommended writer or documented manual procedure.
Is 5GB the universal minimum for a bootable USB?
No. Microsoft specifies at least 5GB free for its documented manual Windows flash-drive method. Other operating systems and image releases can require different capacities, so check the image and publisher instructions.
Should I choose UEFI or BIOS in the boot menu?
Choose the entry matching the firmware mode and installation plan. Use the UEFI entry for a UEFI installation and the legacy/BIOS entry only when legacy boot is required or supported by the plan.
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Do I have to disable Secure Boot?
No. Secure Boot is not a general USB-boot switch. Keep it enabled unless the operating-system or hardware manufacturer documents a specific compatibility problem that requires temporary disablement, and re-enable it afterward when advised.
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Why does Windows say the disk uses the wrong partition style?
Windows Setup may have been started in a firmware mode that does not match the disk’s intended partitioning. Restart and select the matching UEFI or legacy USB entry, then keep that mode consistent through Setup.
Why does a Windows file fail to copy to a FAT32 USB?
FAT32 has a 4GB limit for a single file. If sourcesinstall.wim is larger, use Microsoft’s documented workaround for an oversized file or a supported media-creation path that handles it.
Which key opens the boot menu?
It depends on the computer. Microsoft lists Esc, Delete, F1, F2, F10, F11, and F12 among common keys, and Ubuntu notes that F12 is often used. Check the model’s manual for the exact procedure.
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No. It is an advanced command-line option. A wrong destination can overwrite the running system, so use a graphical writer unless you can identify the whole USB device with confidence.
Why does a Rufus USB fail on a newer computer?
For a particular failure on a modern system that disables legacy compatibility, Ubuntu advises GPT and UEFI (non-CSM). That advice is not a universal setting for every computer, especially one that must boot legacy BIOS.
Does writing the USB erase existing files?
Yes, writing or formatting normally destroys the selected drive’s existing contents. Back up needed files and verify the drive’s model and capacity before confirming the operation.
The Bottom Line
A reliable BIOS or UEFI USB starts with the correct official image and a deliberate firmware plan. Back up the flash drive, identify it by model and capacity, use an image-appropriate writer, and select the matching UEFI or legacy entry at startup. Treat FAT32’s 4GB file limit and Secure Boot as specific technical considerations—not reasons to change settings blindly.
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