ill telll you how i did it, It probably wont matter which distro you use but anyway lets start.
Since you have a seperate hard drive that you dont want to lose data on (if im correct) you're going to want to make a partition on it.
*Go to start
*right-click on computer
*click on manage then click on disk management at the bottom.
*click on your desired hard-drive and shrink it to desired space capacity.
*then right click on the unallocated space and make it a new volume.
*you can assign it a letter but DO NOT format it.
*restart your computer with the mac boot disk in your drive, make sure when it boots you press your "boot from" button
*boot from your disk.
*let the disk boot and an ugly apple screen with an apple and a spinning circle should appear.
*choose your language and then start disk utility in the top bar under utilities(obviously)
*choose your unformatted partition in the left toolbar, make sure its the one on your destination hard-drive
*click erase at the top and choose mac os extended journaled (something like that) from the drop down, then press erase at the bottom.
*get out of disk util when its done
*then press next and click on your partition, it should be the only choice.
*press next then press customize.
*choose anything you will need (cant help you there) and press done then install.
*Have fun!

Since its on another hardrive im not sure how it will boot, you may need to boot from that external hard-drive every time im really not sure, this guide also isn't tailored exactly to you. This is what i used.
I AM NOT RESPONSIBLE FOR ANY DAMAGE DONE TO YOUR COMPUTER MAKE PLENTY OF BACKUPS XD

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• Toshiba l455 series
• 4gbs of RAM
• Intel GMA X4500m graphics card
• pentium dual core processor 2.20 ghz
• 320 gb hard drive
• Realtek RTL8102E/RTL8103E Family PCI-E Fast Ethernet NIC (NDIS 6.20)
• Realtek RTL8187B Wireless 802.11b/g 54Mbps USB 2.0 Network Adapter

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snowosx universal 10.6.4- Everything working except quartzgl and quartz extreme