After updating my Mac to OS X 10.4.10, my external FireWire/USB2 drive didn't mount any more (if plugged in via the USB2 port). I thought about an interface failure specific to my enclosure, but searching on the web, I saw that it was a common problem for 10.4.10 users with Intel Macs and external enclosures. Moreover, I tested the drive on another Mac with 10.4.9 and it worked flawlessly. So it was a problem with the 10.4.10 update.
The only solution found until now was downgrading to 10.4.9 again, but this is a real pain. I was ready to use Pacifist to replace the new USB kernel extensions with the older 10.4.9 ones. But before doing this, I wanted to check if I could force the drive to mount in some way, and accidentally found the workaround.
I used System Profiler to list USB devices and get the VendorID/ProductID of the drive; it was listed amongst my other devices. As I needed some more device information, I launched USB Prober (included with Apple's Xcode Developer Tools) and ... the drive mounted on desktop! Apparently the polling that USB Prober does on USB devices causes OS X to realize that the device is a USB Storage Class product and finally mounts it. I tried several times and it always worked: USB Prober makes the drive mount.
So, if you have the same problem with your external USB device, here's a workaround until Apple fixes it:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070628092633507