An error was reported using the bitcoin client with TrueCrypt:

EXCEPTION: N5boost16exception_detail10clone_implINS0_19error_info_injectorINS_10filesystem
22basic_filesystem_errorINS3_10basic_pathISsNS3_11path_traitsEEEEEEEEE
boost::filesystem::create_directory: Permission Denied: "H:"
C:\Program Files (x86)\Bitcoin\bitcoin.exe in AppInit()

At least in some cases, it seems that a permission error can be returned when an attempt is made to create a directory that already exists if the caller wouldn't have permission to create the directory if it didn't exist. And in some cases, creating the data directory makes no logical sense, for example if the data directory is the root of a USB drive on a Windows system. In these cases, create_directory will throw an exception.

The client should put the create_directory calls in a try/catch block. If an exception is taken, the code should check if the directory exists and, if so, ignore the exception.