I need to compile some apps with the iOS 5 SDK so I discovered I need to upgrade from xcode 3 to xcode 4.
My mac is on OSX 10.6.8, and I downloaded xcode_4.2_and_ios_5_sdk_for_snow_leopard.dmg from my iOS developer account. The image opens fine, does a check to confirm it can be installed, etc. but when I try the install, I get
"the installation failed. an unknown installation error occurred"
How can I fix this problem? Is there a log file to check?
Although this sounds ridiculous, set your computer clock to 1/1/2012 and it will install.
I had exactly the same issue. There is some date / time issue with Xcode's installer.