Bootable USB Query from Newbie

I just got a new machine with 64-bit Mint Cinnamon. It has USB inputs, of course, but no optical drive. I have an external DVD burner that the machine is able to see and use. I want to dual-boot the machine, but before I start that exercise, I want to create some bootable USBs with a couple of Linux distros. I tried for hours to make unetbootin work, and while it will write a lot of bytes to a USB, it doesn't seem to be able to create a bootable one. Using Universal USB on Windows, I was able to create a USB that will boot on my old Debian box, but on my new machine, when I go into the BIOS and attempt a boot from the USB (that I know works), the screen goes black for 5 - 10 minutes and then either loads from the HDD or returns to the BIOS. I can't find any relevant logs in /var/logs. I also tried booting from a DVD using the external drive (that also works once an OS is in place), and it does the same thing. Since the USBs, or at least some of them, work on my Debian box, I think it must have something to do with my BIOS on this new machine, which is Aptio 2.18.1263 from American Megatrends. When I went to their web site, the BIOS version is nowhere to be found. They seem to be promoting Aptio V, whatever that is. Without an optical drive or bootable USBs, I'm not sure how I'd update the BIOS anyway, but it doesn't appear to be an option.

So, can anyone decipher the ramblings above and impart some wisdom about where I'm going wrong or what path I might take to get to the bottom of this? Someone installed an OS on this machine at some point, so it must be possible. I'm just not sure what I'm doing wrong.