The USB sticks are:
1) A LEXAR Jumpdrive 512Mb
2) A 4 GB Sandisk cruzer -- This is one of those devices with the irritating U3 partition, but both windows are opened on the proper contents.
The machine is a "spare parts" box, with an ASUS mother board (I think) with Nvidia nForce chipset.
I did check with pgrep thunar, as per the suggestion in the xfce bug report, and it only shows 1 process.
There doesn't look to be anything out of the ordinary in /var/log/syslog -- but then if there was a problem at that level KDE ought to be affected too.