I am not sure if many people are familiar with this problem, but I'll describe it here anyway and I also offer a solution to recover from Not Formatted state.
I run unslung-able firmware with Maxtor's OneTouch harddisk and I have noticed that if I press shutdown button or power-off my NSLU plugging the power off and then re-starting it, my harddisk isn't being shared anymore. When I go to web interface, it says that disk is Not formatted. I call this a not formatted state, as I had previously formatted my harddisk using Linksys' web interface and even unslinged to it. When I enable telnet and log in (or use ssh if you have dropbear installed) I can see that my disk is mounted almost as it should. I can access it from /share/hdd/data and conf but my passwd file is original Linksys's passwd instead of the that was installed during unsling process. I can see it because I have setup for shell /opt/bin/bash but now my shell is /bin/sh.
I tried to make it work with following procedures:
None of these helped out, I was still on the not formatted state.
Power off your NSLU, disconnect power of external harddisk or plug it off from NSLU and start NSLU.
Wait until it beeps (system loaded) and go to web interface to verify that you disk status is saying Not installed. Then plug in your harddisk (while NSLU is powered on) and wait for it to mount for minute or so.
Then reload your disk management page and you should see that your disk status is saying Formatted now.
Next, telnet or ssh inside your NSLU and type command
After succesfull bootup, you should see status still saying Formatted. Do not use power off button as a shortcut, or recycle power on your NSLU, or you will be back in Not formatted state.
I haven't tried another versions of unslung firmware and I haven't been running with other firmwares or harddisks either, so it might be just an issue with harddisks or just my build version of unslung-able causing this. Publishing this information anyway to help other people who might be suffering of same behaviour.
Same problems as described above but none helps me. After connecting the drive to a workstation and type mkswap /dev/sda3 it works :)