If you have a Lion installer on a USB thumbdrive, you can use it to upgrade your Mac from Snow Leopard to Lion, but if you have an older version of OS X you can’t upgrade. You need to first upgrade to Snow Leopard OR you can erase everything and do a completely fresh install of Lion. After installing Lion on my iMac, I wanted to delete everything on my laptop and do a fresh install of Lion. But the installer will not install over an older version of OSX. I had to delete the old version of OSX, then install Lion from the USB thumbdrive. Here’s how to do a fresh install of Lion.
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