After installing Zorin OS, PC won't boot from USB anymore

Last week we have installed Zorin OS 15.3 core edition on an ACER A315-51 laptop. Now we want to install Debian OS in a dual boot, but the won't boot from a USB drive any more, even though USB HDD is clearly first in the boot sequence menu.
What can we do to solve this problem?

The vast majority of the time in a case like this, the USB is faulty.

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