Didn't you report in another thread that you were unable upgrade Snowy to El Cap on the iMac in question? And if so, I wonder if you'll be any more successful with the bootable installer?
That's correct....for both El Capitan and Yosemite. In both cases, after the install process, the dialogue box said: "Installation Successful" and yet the system was still Snow Leopard.
In the recommendations to use a thumb drive, among the reasons from joemike, were:
The quickest, most reliable, solution would be nuke & pave from a Bootable Thumb Drive installer or external boot drive. Under the circumstances I would be wary of using a Recovery Drive installer in case it too had been somehow compromised.
The bootable thumb drive has the advantage that it boots only a minute subset of macOS, just enough to run DU and the installer, which eliminates version conflicts.
