Following devil's comment, I tried to fix the volume icon on my LaCie. One collateral of reformatting is that you of course lose all the bundled content, but I made a backup of everything and especially the volume icon.
The first thing I tried was simply opening the icon file in Preview, copying it and then pasting to the volume. The result is this.
I'm not sure what's going on here and wtf are these lines, but not pretty at all.
Turns out there's another way to do this, going back to where I started. When I tried to put the ".VolumeIcon.icns" file in the root folder of the mobile hard drive, it had no effect. However, it works fine if I just put it in any random folder on my drive -- this folder then assumes this icon.
So all I have to do now is to view the info of the folder, click on the icon in the top left, copy it, and paste to the mobile hard drive. Works fine.
UPDATE: if you want the actual icon, get it here.Share