Thinkpad T420 won't boot from USB

[Phil Fraser]

I now have my SSD's partition table set to DOS mode (instead of GPT) and 
have Windows 10 installed once again and have shrunk the partition ready 
to install Linux. The problem is that the laptop will not boot from a 
USB stick except for the one that I prepared with Rufus. This, I'm sure, 
has been the root of my problem in getting Windows and Linux to coexist 
on my SSD.

My usual routine is to press F12 and then select USB. This works with 
the Rufus prepared USB stick but no other USB sticks. Selecting USB 
causes the screen to blank for a few seconds and the the menu reappears. 
I also tried setting the first item in boot order to USB HDD. The USB 
stick is ignored and the boot continues from the SSD.

The only way that I can get my Linux prepared USB stick to boot is to 
select "both" (UEFI/Legacy). I think this is how got myself into so much 
trouble, although the installation of Windows and Linux on my mechanical 
disk went without a hitch.

I have searched the Internet for a solution but all that I've found is 
others asking the same question. Obviously it can be done because I'm 
not the only owner of a Thinkpad T420.

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Regards,

Phil