Bootable USB or DVD Reinstall OS on new HDD iMac Mid 2010 21.5"

[Nevis Island]

So I have been working with computers since 1991, but 99.9% Windows and or Linux. I have a friend who has a Imac mid 2010 that I believed shipped with Sierra 10.12. At any rate he hard drive failed completely, (White screen with

blinking question mark) so I replaced the drive. No problem there have replaced thousands of drives.

Now it comes time to install the OS, at first I tried to do it via Internet, but no go on a Mid 2010.

I then create a bootable USB using a dmg file, (which I assume is similar to a ISO file) using TransMac. I also created a Dual Layer DVD the same way. I used El Capitan Sierra, and High Sierra. None of them would boot using C, or

Option.

Also tried clearing NVRAM with P+R+Option+Command

Now here is the STRANGE thing. Just for fun i stuck a bootable Linux Ubuntu USB in and the computer booted and would have installed had I wanted i to. The same with a Windows 8.1 64 Bit pro. :O

Desperately Need help.. Thanks ever so much

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