Here is one possible use for your consideration:
"There is nothing on this thread or any other thread (covering the subject of boric acid) that would indicate even one single practical use for it by terrorists."First off, your chemical formula and example uses sodium borohydride, but that is NOT boric acid. The stolen truck carried boric acid.
Second, even if you managed to dredge up a truck load of sodium borohydride, just where the hell do you expect to come up with a tanker full of the highly toxic gas, boron triflouride, that your formula calls for?
Third, the only terrorist value to the resulting diborane gas is that it is highly combustible. You know like common, ordinary, everyday natural gas and propane that can be gotten anywhere, anytime and with very little cost or trouble. Moreover, your diborane spontaneously combusts with air, which is a highly undesirable characteristic when making a bomb or pyro compound, because it can not be easily controlled and tends to blow up in your face.
Fourth, there are literally hundreds of easier to use compounds, found in ordinary life and business, that are far more useful to terrorists (than what you propose), so that a terrorist does not need to go running around the country side stealing trucks full of sodium borohydride and tankers full of boron triflouride (should they ever find any).
Boric acid is of no practical use to terrorists.
--Boot Hill
Also VERY toxic. Don't know how to describe the odor, except that it smells like copper tastes.