How to install mac os after installing a new hard drive?
Hello, i have changed the hard drive in my mac (Imac late 2009 A1311) how do i reinstall mac os? i tried USB but that did not work, what should i do?
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What do you mean by "i tried USB"?
If you just copied the OS X/macos installer .dmg file to a flash drive, that isn't going to work; it needs to be a bootable flash drive. See Create a bootable installer for macOS - Apple Support You would need another Mac in order to create the bootable flash drive. See also How to download and install macOS - Apple Support
If you still have your old, presumably still working, bootable HDD, you could put it in an external FW800 enclosure, boot from it, download the macOS installer and run it to install macOS on the (new) internal HDD.
The 2009 iMacs could run macOS up to High Sierra (10.13.x).
