After upgrading from Boost 1.34.0 to 1.34.1, I get the Visual Studio 2005 error and building my project: Error 805 error LNK2001: unresolved external symbol "unsigned int boost::filesystem::detail::not_found_error" (?not_found_error@detail@filesystem@boost@@3IA) FolderProcessing.obj I have isolated the error to the declaration: boost::filesystem::directory_iterator it(folder); Using boost 1_34_0 the above declaration worked and with 1_34_0 it fails. I have copied the installed boost 1.34.1 (from Boost Consulting) C:\Program Files\boost\boost_1_34_1 to my project folder C:\projects\ardev\CurrentDev\ExternalLibs\boost where I copied the new boost 1_34_1 release over top the 1_34_0 release. All references to 1_34_0 libraries have been changed to 1_34_1 libraries: ... boost_signals-vc80-mt-gd-1_34_1.lib boost_thread-vc80-mt-gd-1_34_1.lib boost_filesystem-vc80-mt-gd-1_34_1.lib