On Ubuntu, without the locks around the os.system calls, I was not
getting any errors other than file-not-found errors on blah.txt - which
you would expect if another thread had just deleted the file. You didn't
post the errors which your test script was generating - and I assumed
you may have been referring to the file-not-founds on blah.txt.

You definitely need the join calls, as without them atexit processing in
the main thread will close the handlers when the threads are still
running. This will lead to errors like the ones you saw (I/O operation
on closed file), but this is not a bug - you definitely need to join on
all created threads before exiting the main thread - whether test or
production.

Please post the exact errors you are getting, after removing the locks
around the os.system calls. Delete all logthred.log files before the
test run, and please post the console output as Frans did. As I said in
my earlier comment - ensure that anti-virus, Windows indexing and any
other software which may open files at non-deterministic times is
disabled. If you are seeing a WindowsError with an error code of 32
(this might get lost in all the other output, but it's the first error I
found - the other messages look like they are a consequence of that
initial error).