The  UEFI malarkey and Secure boot, Legacy mode (CSM) etc.         

BIOS is now called UEFI or on top brand gaming motherboard, has a nice CSM mode, it is dual EFI and BIOS all in one, best of both words better) Some makers call it DUAL BIOS. (cute no?)

You are here because you can not boot to CD /DVD/BR  optical disks or to USB memory sticks or to external HDD (SSD).
UEFI is not the same as old loved BIOS:
Welcome to UEFI BIOS, (my desktop runs CSM BIOS that is both BIOS and UEFI dual mode (ASUS) .  "Compatiblity Support Module" Or simply of all LEGACY mode.
To boot to USB (or DVD) you must turn off Secure boot and turn on Legacy Support. This first example is a HP PC.(modern )
When you do that on a HP PC, the UEFI sends you  a live secret 4 digit code say, 1234,(this blocks virus attacks, this trick) You then reboot the PC.
The PC tells you next,  omg UEFI security is breached, and asks for the 4 digit secret code, and then you type it, 1234 and does not echo to the screen do not panic, just type it blindly and hit enter.
If you go back into this page in UEFI, you see the below the locked in setting done by me.
I like to put the USB/KEY as 1st boot order. (next) There are special key presses to do that. (seen on the right column) F10 saved all settings and...

YOU can now boot and hammer the F9 key (this is HP boot on the fly key) and then pick USB key or DVD. as the case may be. and even load LINUX.
I will show 2 UEFI screens, one is OEM style, (HP) and the other a top gaming mobo (motherboard by Asus Z270-p)
Old PCs are easy , power on, hammer the F9 key to  boot to USB and then hammer (tap,tap,tap...) the space bar to satify the El Torito spec rules, if booting external media of any kind. (see hybrid here)
On a newer PC's not so easy this is. (yoda said) but yes it is, flip 2 bits bam legacy.
See how to move order of USB to the top, using prompted keys in the right window.

This PC has CSM BIOS, (does it all !) I turn on CSM first, then advanced then legacy all.  Bam it boots to anything I want. This is my Z270p motherboard in action.

Doing this you can boot to USB and...
  • CD
  • DVD
  • BD (blue ray)
  • Even SD memory cards can boot, like my HP does.
  • Or PXE Network boots from its server.
  • HDD
  • SSD

Side issues on UEFI:
Also turn off fast boot if you want to use linux as dual boot or use linux to back up the windows files latter (like windows is dead?). (or you will be sorry) W10 it's called Fast Startup.
If you forget to do that running Linux , at the least Linux sees C: as missing or illegally formatted, saving your bacon. (tricky no?)
UEFI can be just wrapper around your normal BIOS code.  (a shell  of protection from virus infections)
UEFI allows booting any HDD over 3TB and GPT formatted drive. (most BIOS can not do that, and expections exists for BIOS +GPT, etc...)
MBR disks can not cross, over, 2.2TB (approx) size. ever. So GPT is the only path and BIOS that supports it , is called UEFI. 
There are execeptions to this rule, special HDD(that shrinks with a jumper) or new SATA PCI-e card with its OWN BIOS EXTENSION. ( yah the beauty is that word extensions, that IBM invented) but MBR format  is limited no matter what you do.
See this trick on big server. HP Generation 8 server.
MBR = 32^2 (
4,294,967,295) Times 512byte sectors, so is limited to 2.2TB (do the math or wiki MBR) 32 to the 2nd power.
Most of us now boot from small, SSD (only speed matters to me), and we don't have $3000 for 3TB SSD, nor wish to. (in fact I am booted from M.2 screaming fast SSD)
Also you need 64bit OS, to run huge  HDD, and for sure USB external HDD this large.(and chips inside that work, some don't)
My Z270p mother board can not boot to any GPT drive in CSM mode,but can use them all day long, in windows, 64bit. (just can't boot from GPT in CSM mode but sure can in  UEFI )

A UEFI/BIOS video, that is pretty accurate:(hit stop at 4;00 minute mark)

version 1.  8-8-2018