Knowing that the problem is related to the internal representation of the strings, I have written a short script which reproduces the problem. It is this simple:

import os
name = 'sub-fcc'
wrkdir = 'D:\\Bug reports\\Python\\test'
dirname = wrkdir+os.sep+name
print(dirname)
print(ascii(dirname.encode("unicode_internal")))
dirname += os.sep+'bulk'
print(dirname)
print(ascii(dirname.encode("unicode_internal")))

Output:

D:\Bug reports\Python\test\sub-fcc
b'D\x00:\x00\\\x00B\x00u\x00g\x00 \x00r\x00e\x00p\x00o\x00r\x00t\x00s\x00\\\x00P\x00y\x00t\x00h\x00o\x00n\x00\\\x00t\x00e\x00s\x00t\x00\\\x00s\x00u\x00b\x00-\x00f\x00c\x00c\x00'
D:\Bug reports\Python\test\sub-fcc\bulk
b'D\x00:\x00\\\x00B\x00u\x00g\x00 \x00r\x00e\x00p\x00o\x00r\x00t\x00s\x00\\\x00P\x00y\x00t\x00h\x00o\x00n\x00\\\x00t\x00e\x00s\x00t\x00\\\x00s\x00u\x00b\x00-\x00f\x00c\x00c\x00\x00\x00\x00\x00\xd8\xa3\x90\x02\x00\x00'

The end of the output varies from run to run.

It works correctly in Python 3.2.3 64-bit on Windows 8.