Member Since: Oct 01, 2007
Mac Specs: 27" iMac i7 (2011) 10.10, iPhone6, iPad Air 2, AppleTV2
05-12-2010, 07:44 AM
Assuming you're taliking about installing on the bootcamp partition not a Virtual Machine:
You aren't forced to back up, it's a personal choice.
The question you have to ask is do you have data you don't mind losing if the whole machine goes belly up?
If the answer is no, then there's no need to back up.
As for installing from an external HD(it needs to be bootable) and needing the snow leopard disc (officially, yes) then I'd read this:
External hard disk acquisition addict - but admitting the problem is the first step to a robust backup
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