Strange USB boot problem, Lenovo T450S

I've (quite accidentally) stumbled on a strange problem with USB booting 
on this machine. It seems that there are some flash drives that the 
machine will not boot from. For instance, I have a 4GB Verbatim Store 'n 
Go which has MX 19.3 on it. That works fine, boots, allows me to 
install. I have another which allowed me to install Windows 10. But 
there are some disks which don't work at all.

I have several Integral flash drives, one quite old, some recently 
bought. The T450 won't boot from any of them. If I try, it accesses the 
flash drive, the screen goes blank and I might just as well switch it 
off. Can't use CTRL-ALT-DEL no response. Power switch is the only 
option. Applies even if I write the flash drive on the T450. But, the 
same flash drive will boot any other machine I've tried it on.

I've also hit a problem with Ventoy, even on a flash drive that the 
machine will otherwise boot from. Installed the latest version of Ventoy 
to the flash drive from the T450, copied several iso files to it. When I 
attempt to boot the T450 from that flash drive, the initial menu comes 
up, but anything that I select from the menu has the same effect as 
booting from one of the Integral drives. Blank screen, power off the 
only option. But the same drive will boot any of the isos in several 
other machines,

I've never seen anything like this before. Anyone have any idea what 
might be the problem? I'm on the latest firmware, and everything works 
fine otherwise.


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