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I inserted Mountain Lion on an iMac A1225 having new formatted HDD. When I boot holding the ⌥ Option, it shows Mountain Lion (install disk) as option. When I press enter-key to confirm, it came with "no bootable device" but if I boot holding "C" it boot Mountain Lion normally. Now with the new OS installed, the iMac still gives "no bootable device" at start up

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If you can get Lion to boot you can go to the Startup Disk in System Preferences and select the internal disk. Maybe it in Recovery Mode if it won’t boot.

https://support.apple.com/en-us/HT202796

Basically does this under the hood.

To bless a volume with only macOS or Darwin, and create the BootX and boot.efi files as needed:

bless --folder "/Volumes/Mac OS X/System/Library/CoreServices" --bootinfo --bootefi

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