Do you have a solution for this same problem in Mountain Lion?
Unfortunately selecting the icon and hitting delete no longer works.
Alternatively if I could find the directory that contains this icon I could just drag it into place from there.
If anyone has these answers I would appreciate the help.
> Click Finder
> Click Go
> Click "Go to Folder..."
> Enter "/System/Library/CoreServices/CoreTypes.bundle/Contents/"
> Look for "GenericTimeMachineDiskIcon.icns"
> Drag icon file while holding option onto icon in top left of Hard Drives "Get Info" window.