Failure to boot from external USB thumb drive

I raised this query way back in March 2021 and have now found a partial 
solution, in that I can boot the T450s from a USB drive as long as it 
is NOT connected to the dock. As you can possibly imagine this is a 
"pain in the butt".  So my question is:  Has anyone else had this 
problem and, even more important found a solution for it,please?

Zed

on a very warm Spring afternoon in New Zealand
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