The BIOS/UEFI provides the basic functions needed to boot your machine and to allow your operating system to access your hardware. Your system provides a BIOS/UEFI setup menu, which is used to configure the BIOS/UEFI. To enter the BIOS/UEFI setup menu you have to press a key or key combination after turning on the computer. Often it is the Delete or the F2 key, but some manufacturers use other keys. Usually upon starting the computer there will be a message stating which key to press to enter the setup screen.

Within the BIOS/UEFI setup menu, you can select which devices shall be checked in which sequence for a bootable operating system. Possible choices usually include the internal harddisks, the CD/DVD-ROM drive and USB mass storage devices such as USB sticks or external USB harddisks. On modern systems there is also often a possibility to enable network booting via PXE.

Depending on the installation media (CD/DVD ROM, USB stick, network boot) you have chosen you should enable the appropriate boot devices if they are not already enabled.

Most BIOS/UEFI versions allow you to call up a boot menu on system startup in which you select from which device the computer should start for the current session. If this option is available, the BIOS/UEFI usually displays a short message like “press F12 for boot menu” on system startup. The actual key used to select this menu varies from system to system; commonly used keys are F12, F11 and F8. Choosing a device from this menu does not change the default boot order of the BIOS/UEFI, i.e. you can start once from a USB stick while having configured the internal harddisk as the normal primary boot device.

If your BIOS/UEFI does not provide you with a boot menu to do ad-hoc choices of the current boot device, you will have to change your BIOS/UEFI setup to make the device from which the debian-installer shall be booted the primary boot device.

Unfortunately some computers may contain buggy BIOS/UEFI versions. Booting debian-installer from a USB stick might not work even if there is an appropriate option in the BIOS/UEFI setup menu and the stick is selected as the primary boot device. On some of these systems using a USB stick as boot medium is impossible; others can be tricked into booting from the stick by changing the device type in the BIOS/UEFI setup from the default “USB harddisk” or “USB stick” to “USB ZIP” or “USB CDROM”. You may need to configure your BIOS/UEFI to enable “USB legacy support”.

If you cannot manipulate the BIOS/UEFI to boot directly from a USB stick you still have the option of using an ISO copied to the stick. Boot debian-installer using Section 4.3, “Preparing Files for Hard Disk Booting” and, after scanning the hard drives for an installer ISO image, select the USB device and choose an installation image.