If you've converted your USB flash drive to a recovery disk or live USB, you can boot to the media any time you need to repair or make changes to your system. Depending on how your computer is set up, however, it may fail to recognize the USB drive as a bootable device. To troubleshoot the issue, check your system configuration using the Unified Extensible Firmware Interface -- or BIOS, for an older PC -- and, if necessary, reload the boot files onto the flash drive.
Make sure the flash drive is connected directly to the computer and not to a USB hub. USB hubs often fail to provide sufficient power to external drives. Similarly, try connecting and then reconnecting the drive, or inserting the device into another USB port. If your computer still fails to boot to USB, connect the removable media to another computer to see if the issue is with the original PC or with the drive itself. If your computer is an old model, it may lack the ability to boot to a USB drive.
Change the Boot Priority
For Windows 8 or 8.1, click the Power button on the Settings charm ("Windows-I"), and then press and hold the "Shift" key while clicking "Restart." Your computer will boot to the Advanced Startup Options. Click "Troubleshoot," click "Advanced Options" and then click "UEFI Firmware Settings" to access the UEFI. On Windows 7 and earlier versions, restart the PC and then follow the instructions on the boot screen to go to the BIOS. Typically, on the Boot tab or Advanced Bios Features screen, you'll see the option to select a primary boot device. If you highlight the option and press "Enter," setup will display a list of devices from which the computer can boot. Using the cursor keys, select "USB-HDD" or the name of your device, then press "Enter."
Enable the USB Controller
Some versions of UEFI or BIOS allow you to control the behavior of peripheral devices you connect to your PC. If the USB controllers used to detect and manage USB devices are disabled, your computer won't properly recognize the flash drive. On the Advanced tab or Integrated Peripherals screen, search for "USB Controller" or a similar option and use the "+" or "-" keys to change the setting to "Enabled." System setup occasionally but rarely includes the option to toggle USB storage on and off. If your system contains the option, confirm it's set to "Enabled" as well.
Reformat and Try Again
If your computer still can't boot from your flash drive even after you correct your system settings, or if the problem persists across multiple PCs, you may have unintentionally corrupted the data. Reformat the disk and then reload the boot files onto the device. Before you disconnect the flash drive from your computer, expand the notification area and check for the Safely Remove Hardware icon, which looks like a USB connector with a green check mark next to it. Don't unplug your flash drive from your PC unless you see this icon in the system tray; otherwise, you run the risk of data loss or corruption.
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