I'll try to be as clear as possible, being a Linux geek, not an OSX one!
After struggling me too with the "Error 0x1FFFFFFF" about my external LaCie, I made it to mount it, but working from the Terminal, not from its panel in SystemPreferences (there, the mount button still gives that damn error).

The way to overcome the error was simply to do it as root: as a normal user I was prevented to mount it.

Now, the current problem is this: although it is mounted, and accessible from Terminal, ad visible in "df", it doesnt appear as an icon, nor it is present in the /Volumes dir!
Probably due to some strange way to manage the /Volumes dir by OSX, I thought, so I mounted it under a folder in Desktop, but it was even worse, arriving to the point of making the folder disappear when clicked...

possibly it's just me, not understanding some simple OSX rule - can you help???