BIOS won't boot Debian from USB

I know this is a common problem that has likely been addressed here before, but thus far none of the obvious solutions have worked. I'm trying to install Debian on my laptop (currently running Ubuntu) from a USB flash drive. I have reformated the drive to the FAT32 file system and made sure it has only one partition, I have used Unebootin to create the proper files on my flash drive, installed the ISO image on my root directory and the USB drive, programmed my BIOS chip to boot from the USB (put the HD last in boot order), and disabled secure boot and fast boot; but it just won't boot from from the USB drive. Instead it just boots directly to Ubuntu as if that was the only choice available. I've even tried pressing F2 during the boot sequence and have also tried using both USB ports on my computer, but to no avail. If I have to, I suppose I can live with Ubuntu, but I've been married to Debbie over a decade now and really don't feel like getting to know a whole new OS - even if she's related. Any hints or suggestions would be much appreciated. Thanks.