Failure to boot from external USB thumb drive

My problem is similar to that outlined by Paul Marwick 
<<20210330190828.bxhjqygbpn5qldbm@blynas> but there are sufficient 
differences that I do not want to hijack that thread.

I have a Lenovo Thinkpad T450s running Linux Mint v20.1 Mate from a 
246GB SSD. It is set up, to boot in Legacy mode.

The laptop is my only computer and is attached to a dock and,in addition 
to the internal SSD, I have an external 246GB SSD,used as a Backup drive 
and an external 500GB HDD,used as a Data drive.Both are connected via 
USB ports. There is a USB keyboard and mouse and a AOC 23.7" VGA Monitor.

There is no problem accessing external USB thumb drives containing 
audio,data or video but there is no success booting an ISO.

I follow the approved sequence.

Boot the computer and Press Enter and when asked choose F12 to go into 
the Options menu

The options menu appears but there is no choice to boot from the 
external USB thumb drive. I have used Adata, Emtec, and Sandisk 
drives,without success.

I have tried various combinations in BIOS - Boot only in Legacy 
mode,Boot in either UEFI or Legacy Mode. I know I can change my boot 
file to UEFI but as this would entail a new install of a very much 
modified Linux Mint,I am reluctant to do so, unless someone can 
guarantee that I will then be able to boot an ISO from a USB thumb drive:-)

If I burn the ISO to a DVD drive and attach an external DVD to the 
computer,the option to boot from the DVD appears and I have no trouble 
booting. The major problem,of course,is this methods is so slow :-(

So,this ancient old man is seeking a solution to the problem,having 
searched the internet for guidance without finding the answer.

zed


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