Low-tech method, but one that's worked for me in the past: run HD SC Setup again.

I had an old SE with this problem, and when I tried reformatting the HD it didn't work, and the HD wouldn't mount on the desktop. Rebooted, went into HD SC Setup again, then tried formatting the drive again (HD SC Setup could still "see" it even though it reported it as unmounted, with no name and no filesystem). When I did it this second time, it was successful.

M