Thanks for your fast response!

I updated the "ServerExec" line to "Exec" as per the link you provided, but there's still no /tmp/test file getting created on boot.

Code:
knoppix@Microknoppix:~$ cat .config/autostart/startup.desktop 
[Desktop Entry]
Name=Startup
Exec="/bin/mkdir /tmp/external"
Icon=
Type=Application
If there's errors with processing .config/autostart/* files, is there anywhere these errors will be logged so I can check for messages?