The harddrive method is not checking if the image exists before trying to mount it. 

Traceback:

Traceback (most recent call last):
  File "/usr/lib/anaconda/gui.py", line 1253, in handleRenderCallback
    self.currentWindow.renderCallback()
  File "/usr/lib/anaconda/iw/progress_gui.py", line 245, in renderCallback
    self.intf.icw.nextClicked()
  File "/usr/lib/anaconda/gui.py", line 951, in nextClicked
    self.dispatch.gotoNext()
  File "/usr/lib/anaconda/dispatch.py", line 146, in gotoNext
    self.moveStep()
  File "/usr/lib/anaconda/dispatch.py", line 217, in moveStep
    rc = apply(func, self.bindArgs(args))
  File "/usr/lib/anaconda/backend.py", line 160, in doInstall
    backend.doInstall(intf, id, instPath)
  File "/usr/lib/anaconda/yuminstall.py", line 901, in doInstall
    self.ayum.run(self.instLog, cb, intf)
  File "/usr/lib/anaconda/yuminstall.py", line 424, in run
    self.method.switchMedia(i)
  File "/usr/lib/anaconda/harddrive.py", line 114, in switchMedia
    self.mountMedia(mediano)
  File "/usr/lib/anaconda/harddrive.py", line 48, in mountMedia
    isoImage = self.isoDir + '/' + self.path + '/' + self.discImages[cdNum]
KeyError: 3

Obviously the third image is not present :-)

Attaching a (untested) patch which should give a slightly nicer user experience
(assuming SystemError is caught and handled appropriately).
Even better would be to actually check if /all/ required images are present
before allowing the user to continue with the installation.