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I have a SL 10.6.8 VBox running as we speak. It can be interesting to disable extra language translations or printer support (you save about 2 GB). Before that, you can choose to customize the installation. Now open Disk Utility under Utilities menu. The installation will start after some minutes. Open VirtualBox and power on the virtual machine.
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After the last instance of ExtraDataItem tags add the following two, new, ExtraDataItem tags:
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Search this file in a text edit for the ExtraDataItem tags. This will be in ~/Library/VirtualBox/Machines//.xml. Find the XML file that defines the virtual machine that you just created.
HOW TO INSTALL OS X 10.6 ON VIRTUALBOX ISO
Open the settings for this new virtual machine, and in the Storage section, load the Snow Leopard ISO in the now empty CD/DVD device.Create a new virtual hard disk, I chose 20 GB of dynamically expanding storage. Name it as you want, but select "Mac OS X" as the operating system, and "Mac OS X Server" as the version. In VirtualBox, create a new virtual machine.Rip your original retail DVD of Snow Leopard in ISO format.Taken from here - abridged and modified for OS X guest OS: These instructions are for VirtualBox on Windows and Linux but they work well on OS X as well.
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