I was going to say have you checked your drive setup, but since you've had it working with Leo4All this probably won't apply.

If your hard disk is ide, make sure that the CD drive is set to master on channel 0, with the hard disk setup as slave on the same channel. On my machine I was getting the same error until i did this.

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 Mac OS X 10.6.4 Retail || Intel Core2 Duo E8200 2.66GHz || Gigabyte G31M-ES2L || GeForce 6600 GT 128MB || Realtek ALC883 Audio || Realtek RTL8169 LAN || Samsung SpinPoint F1 1TB HD || Sony Dual Layer IDE DVD RW DW-D22A || Apple Aluminium Keyboard || Mighty Mouse

 MacBook Pro || 2.4Ghz Core 2 Duo || 200Gb HD || 2Gb Ram || NVIDIA GeForce 8600M GT

 iPhone 4 || 16Gb