Bootable USB flash drive not detected by the BIOS

Solution 1:

Some BIOSes only re-enumerate USB drive 'types' on a cold boot. This means that if you change the partition\format structure of the USB drive, on some systems you must power them off and on again. If the system always stays on and you use crtl-alt-del or restart to begin a USB boot session, the BIOS may not 'notice' that you have changed the 'type' of partition. For instance, Windows will format a raw, unformatted USB flash drive as a floppy drive (no MBR). This won't MBR-boot correctly. If you use a new partition/format tool to change the partitions to a HDD MBR type of drive, the BIOS will still treat the USB drive as a USB:FDD drive and not a USB:HDD drive unless you reset the CPU or power-off and in again. This type of BIOS is rarely found these days, but it is worth remembering this 'gotcha'!

Solution 2:

Possibly your USB Stick is recognized as a HDD instead of as a USB device.

In you bios menu, when you pick the boot priority, you have the option to chose between USB, HDD and FDD drives. However, additionally, you can select the HDD to boot from in a separate menu on the boot menu page. Go into that menu, and check if your USB stick is listed next to the HDD drives in your system.