Hi!
I want to reinstall OSX on my late 2009 iMac. I don't have the installation CD but think I can use an external USB HD? Today I run OSX Yosemit, 10.10.2. It would be very helpful to get an step by step instruction.
iMac (27-inch Late 2009), OS X Yosemite (10.10.2)
Are you trying to perform a clean install of Yosemite (10.10) - that is do you want to erase the hard drive and start over? Or do you merely want to install 10.10 on top of the OS you already have?
I want to erase the hard drive and have a fresh new start.
You'll lose everything if you don't have a backup. Check this article. OS X: About OS X Recovery - Apple Support
Reinstall OSX on late 2009 iMac.
