I tried to use NoPriv (https://github.com/RaymiiOrg/NoPriv/) to backup emails to an USB Disk which is formatted for Windows compatibility, i.e. FAT-type filesystem, but got the error below. Seems mailbox.py still tries to create a symbolic link, which is not possible on this filesystem.

Running on ext3 file system works fine. This is on Ubuntu Precise 12.04 with latest updates installed. Python is 2.7.3-0ubuntu3.1

Getting messages from server from folder: [Gmail]/Sent Mail.
Saving message 1.
Traceback (most recent call last):
  File "./nopriv.py", line 783, in <module>
    get_messages_to_local_maildir(folder, mail)    
  File "./nopriv.py", line 236, in get_messages_to_local_maildir
    saveToMaildir(raw_email, maildir_folder)
  File "./nopriv.py", line 155, in saveToMaildir
    message_key = folder.add(msg)
  File "/usr/lib/python2.7/mailbox.py", line 276, in add
    os.link(tmp_file.name, dest)
OSError: [Errno 1] Operation not permitted

The problem is that mailbox decides if links are possible based on the OS rather than on the file system.  This is a general problem that Python has...when the Python file system APIs, and this code, were written, OSes could only read their own file systems, and there was only one file system type per OS.

Fixing this, then, pretty much requires solving the general problem of introspecting file system capabilities.  I think there are open issues relating to that.