Start with the firmware boot menu, not GRUB. If the USB does not appear there, the problem is usually a disabled USB-boot setting, the wrong port, a faulty flash drive, or media that was written incorrectly. If the USB is listed but the computer skips it, select the UEFI entry, recreate the stick from a fresh ISO, and make the installer’s boot mode match the PC’s existing installation. A Secure Boot violation is a separate trust-chain problem and needs its own fix.
This guide takes you through those checks in the order that prevents wasted reinstalls or accidental erasure of an internal drive.
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First, identify which failure you have
Watch what happens immediately after you choose the USB. The symptom determines the branch to follow:
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| No USB entry in the firmware boot menu | Detection, port, firmware setting, or bad media | Try another port and a known-good stick; then check whether external-boot support is enabled. |
| The USB appears, but the PC starts Windows or the installed Linux system | Wrong boot entry or boot order | Use the one-time boot menu and choose the entry explicitly labelled UEFI when the PC uses UEFI. |
| The USB is listed, but selecting it returns to the internal drive | Incorrectly written image or a firmware-mode mismatch | Recreate the media from a fresh ISO and match UEFI or BIOS/CSM to the installed system. |
| A Secure Boot violation, invalid signature, or similar trust error | Boot components are not trusted by firmware or the distribution’s shim | Test with Secure Boot temporarily disabled, or use a distribution image with a valid signed boot chain. |
| The live environment begins loading and then freezes | Corrupt media, a problematic port, or graphics compatibility | Recreate the stick, change ports, and select the distribution’s safe-graphics or compatibility option if offered. |
Do not change partitions on the internal disk while you are still diagnosing USB detection. A stick that is absent from firmware cannot be repaired by editing GRUB inside the installed operating system.
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1. Confirm that the firmware can see the USB
Open the one-time boot menu
- Shut the PC down completely.
- Insert the USB before powering on. Some firmware only scans external devices during the initial hardware check.
- Power on and use the manufacturer’s boot-menu key. The key varies by model; the screen may identify it as Boot Menu, Boot Override, or a similar option.
- Choose the USB device from the list. If two entries for the same stick appear, select the one beginning with or explicitly labelled UEFI when the installed system is UEFI-native.
A one-time selection is safer than immediately changing permanent boot order. It tests the stick without making the internal disk cease to be the default on the next restart.
Enable external boot if the device is hidden
Enter the full UEFI setup when the stick is not listed. Manufacturers use different labels, including External Device Boot, USB Boot, Removable Media Boot, Boot Override, CSM, and Legacy. Enable USB or external-device boot, save, restart, and open the one-time menu again. If a supervisor password or corporate policy locks that setting, the computer’s administrator or vendor documentation is required; do not guess at security settings.
Keep the USB connected directly to the PC while testing. A hub, adapter, or front-panel port can add another failure point. If the firmware still shows nothing, move to the media and hardware checks below rather than changing Linux boot files.
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2. Recreate the USB from a fresh ISO
Download and verify the image before writing
Obtain a fresh ISO for the distribution and edition you intend to boot, saving it to the computer first. Ubuntu’s desktop documentation specifically warns against downloading the ISO directly to the target USB. A fresh download removes a partially transferred or damaged image from the list of possible causes.
Writing an image is destructive: every existing file on the selected USB is erased. Copy personal files elsewhere, disconnect other removable drives if practical, and identify the target by its capacity and model before pressing the write button. Selecting an internal disk would destroy its partitions.
Use image-writing mode, not ordinary file copy
- Launch a trusted boot-media writer.
- Select the downloaded ISO as the source.
- Select the intended USB by its device name and capacity.
- Start the write operation and accept the warning that the drive will be erased.
- Wait for the writer to finish and safely eject the stick before reinserting it for a boot test.
Dragging an ISO file onto an otherwise empty flash drive does not create the boot structures firmware expects. The utility must write the image sector by sector or use its equivalent direct-image function.
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Rufus settings for a modern UEFI PC
If you use Rufus and a modern PC still ignores the result, Ubuntu recommends these choices:
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- Partition scheme: GPT.
- Target system: UEFI (non CSM).
- Write method: choose ISO mode if Rufus asks how to write the image.
Those settings are appropriate for a UEFI-native installation. They are not a universal answer for an older BIOS-only computer or for a dual-boot setup whose existing operating system was installed in legacy mode; use the matching-mode guidance in the next section.
3. Match UEFI and BIOS/CSM to the installed system
Why the same ISO can show different boot paths
UEFI and legacy BIOS/CSM are different firmware boot paths. A single USB can expose both, so the boot menu may show a plain device name and a second entry prefixed with UEFI. Starting an installer in one mode and installing alongside an operating system that uses the other can produce a system that boots only when the firmware mode is changed manually.
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Fedora’s guidance states: The Golden Rule of UEFI: Install Fedora in the same way as the currently-installed operating system(s) is/are installed.
In practical terms, a UEFI-native installation normally uses GPT, while a BIOS-native installation normally uses an MBR-style layout. The installer’s boot mode, not only the disk’s partition table, determines which bootloader path is created.
Choose the correct entry for dual boot
- Determine whether the existing operating system starts in UEFI or legacy BIOS mode using its firmware information or the operating system’s own documentation.
- Restart with the USB inserted and open the one-time boot menu.
- Select the USB entry with the same mode. Choose UEFI: [USB name] for a UEFI installation; choose the non-UEFI entry only when the existing installation is intentionally legacy BIOS/CSM.
- Inside the installer, confirm that its reported boot mode agrees with that choice before modifying any disk.
If your goal is only a live session, consistency still matters: launching the stick in the wrong mode can hide expected boot entries or make a later installation use an unwanted layout.
4. Resolve Secure Boot violations
What Secure Boot is checking
Canonical’s Ubuntu Security documentation defines UEFI Secure Boot as a security mechanism that prevents untrusted code from executing during system boot.
On supported Ubuntu media, firmware starts a Microsoft-signed shim; that shim validates Canonical-signed GRUB and kernel components. If the image contains an altered, unsigned, or otherwise untrusted component, firmware or shim can stop before the live environment appears.
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Use the error message to choose a remedy
- Secure Boot violation, invalid signature, or security policy error: suspect the trust chain. Recreate the stick from an unmodified distribution ISO first.
- Unsigned custom kernel or remastered image: the image may require its own enrolled keys or a different trust policy; the distribution’s normal signed-media path may not apply.
- Known-good signed image still rejected: check the firmware date, Secure Boot key state, and vendor guidance before assuming the USB itself is defective.
Temporarily disable Secure Boot as a diagnostic
- Enter UEFI setup and locate Secure Boot; the setting may be under Security, Authentication, or Boot.
- Record the original value and any key-management state.
- Disable Secure Boot temporarily, save, and try the USB once.
- If the stick now starts, the change identifies a trust-policy conflict. Re-enable Secure Boot for normal use and switch to signed media or the appropriate enrolled-key procedure rather than leaving security disabled without a reason.
Disabling Secure Boot does not repair a missing boot entry or a badly written image. It is a controlled test for a signature problem.
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Use a simple hardware test matrix
| Test | Interpretation |
|---|---|
| Same USB in a different built-in port | If it appears, the original port or its power path is implicated. |
| USB connected without a hub or adapter | If direct connection works, the hub or adapter is incompatible during early boot. |
| Known-good spare flash drive written with the same ISO | If the spare boots, the original stick or its write process is suspect. |
| Original stick on another computer | If it fails there too, replace the stick or recreate the image; if it works elsewhere, investigate the first PC’s firmware and ports. |
These comparisons separate a defective drive from a firmware or ISO problem without touching the internal disk. Official Ubuntu and Fedora guidance supports recreating media and checking firmware, but no vendor behaves identically, so a particular port, hub, or firmware revision may still have its own limitations.
When no USB device is detected at all
Check that external boot is enabled, that the port supplies power during pre-boot, and that the PC’s firmware is current according to the manufacturer. Do not flash firmware as a first reaction to one failed stick; establish whether a second, known-good USB is also invisible. If every removable device is absent, the issue is more likely firmware policy, hardware, or port power than Linux boot files.
6. Fix a boot that starts and then freezes
Rebuild before changing kernel options
A freeze after the logo or initial messages can result from a damaged image just as easily as from graphics hardware. Write the ISO again, use a different port, and test a spare stick. This removes the simplest variables before you alter boot parameters.
Try the distribution’s compatibility entry
Many live boot menus provide Safe graphics, Basic graphics, or another compatibility option. Select it when the normal entry freezes and compare the result. The exact kernel parameter depends on the distribution, graphics chipset, and release; there is no single option that is safe to prescribe for every Linux image. If compatibility mode works, install or update the graphics stack from within the distribution after confirming that the media itself is sound.
Separate a display problem from a complete lock-up
Look for signs that the system is still responding: keyboard LEDs, a changing cursor, or a console-switch key sequence documented by the distribution. If only the display is frozen, the graphics path is a stronger suspect. If the machine powers off, resets, or stops responding to all input, return to the image, port, and firmware tests instead of repeatedly changing graphics parameters.
Quick Recap
A complete, low-risk troubleshooting sequence
- Back up files on the USB and any data you might touch during installation.
- Insert the stick before power-on and open the one-time boot menu.
- Enable external or USB boot if the firmware hides removable media.
- Choose the UEFI-labelled entry when the existing installation is UEFI-native.
- If the stick is absent or loops back to the internal drive, download a fresh ISO to the computer.
- Write the ISO with a direct-image tool, verify the target device, and accept the erase warning only for the intended USB.
- For a modern UEFI PC using Rufus, select GPT, UEFI (non CSM), and ISO mode when prompted.
- For dual boot, launch the installer in the same firmware mode as the operating system already on the disk.
- If a signature error appears, test once with Secure Boot disabled; then restore the preferred security policy and use a signed image or correct key configuration.
- If loading freezes, test another port and spare stick, then use the distribution’s safe-graphics option.
What not to do
- Do not copy the ISO as an ordinary file and assume the USB is bootable.
- Do not erase or repartition the internal disk merely because the USB is missing from the boot menu.
- Do not force legacy mode on a UEFI installation, or UEFI mode on a deliberately legacy installation, when setting up dual boot.
- Do not leave Secure Boot disabled permanently just because it helped isolate one error; restore it unless your chosen image and security design require otherwise.
- Do not keep retrying the same stick through the same hub when a spare drive and direct port can distinguish the fault.
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