I was too quick with responding and forgot about one simple thing you can check... Could you try installing one of those lighter window managers (let's say wmaker), start it via GDM and then run:

nautilus --no-desktop

from the terminal and then plug in the memory stick? Does the error message appear then? Also note if there are any error messages during nautilus start (if so, please paste them here or send them to me by e-mail)

I also forgot to ask about the memory itself.
1. What kind of file system it has?
2. Does it contain encrypted containers or is it encrypted at all?
3. Does it mount without problems on other machines?
4. Do you have different memory sticks to check them with your setup?

It would be great if you could join #gnewsense-dev channel on FreeNode (if you have the time and possibility). "Realtime feedback" could bring some refresh to my damaged brain ;)

(Sorry for the noise...)

Yes, automatic mounting of USB is what I meant.

ad 1) What do you mean by "segfaults in the background"? I am not sure how to look for those.
ad 2) Yes, this is indeed possible.
ad 3) No, no relevant messages appear in dmesg.

Yes, GNOME is started via GDM. Unfortunately, there is nothng in .xsession-errors' file after mounting fails.

The user is already a member of "plugdev" group.

Any more ideas about how to troubleshooot this?

(from rms)

In the new gNewSense, I often get a dialog window with an error which says "Not authorized to mount".

If I try the same memory again, it works. In fact, if I simply take it out and reinsert it, it works.