Okay, I've just done this again on 2.3.1. For years I've searched around the internet for a step by step guide to enabling usb and setting it as internal storage. Not sure if that is what you want but anyway....
1) Run the free patchstick (
http://code.google.com/p/atvusb-creator/)
2) From software menu install NitoTV
3) From NitoTV go to settings and install software, and then smart installer. No need to copy the files over, it will download them now, takes 20 mins or so.
4) make sure the HDD is formatted.
5) Restart the appletv and when the movie plays plug the HDD in.
6) I'm doing this in windows but commands are the same in terminal;
Open putty (terminal)
sudo defaults write \
then
/Library/Preferences/SystemConfiguration/autodiskmount \
then
Automountdiskswithoutuserlogin -bool true
then
sudo reboot
this should automount USB at startup
If you want the next bit to get it seen as internal storage let me know and I'll copy that out.
Hope this helps.
Gaztov