reFind does not detect Mac OSX boot medium [USB]

[Myriel Linn]

Hello,
I've written a Mac OS Sierra .iso on an external hard drive via dd (overkill, but I didn't have a large enough USB Stick on hand), which I am trying to boot in reFind to reinstall Mac OS back on the Mac (mid-2011), which has been running only Fedora Linux for the last two years.
The medium is correctly detected by Fedora itself, but does not show up at all in the reFind menu.
I tried enableing all possible scan flags in the config, but even that did not result in a detection.

I would gladly take any advice in how to proceed to install Mac OS back on the computer.

(If the question arises why I don't use the Mac's recovery: I tried, but it's telling me that my computer has no hard disk on which the system could be installed, which does confuse/concern me a bit.)

[Roderick W. Smith]

Try hitting the Esc key in rEFInd. Sometimes external media are slow to be detected, and hitting Esc causes rEFInd to re-scan boot media, which often works around the problem.

You might also try installing rEFInd's ISO-9660 driver. Without that driver, most EFIs will read nothing but the El Torito image on an ISO-9660 disc. When written to a hard disk, the EFI won't read the El Torito image. If the .iso file uses ISO-9660, though, it may be readable (and bootable) when the ISO-9660 driver is loaded.

Another possibility is that the .iso file you used simply was not meant to be used when written to a USB drive, at least not in the way you've done it. MacOS includes tools, such as Disk Utility, to copy disk images to disks. You may need to use one of these tools rather than dd.