iMac trouble booting from SSD

I have just installed a new SSD into my iMac. My problem is that the main SSD is running Mojave, but I want to run El Capitan, as that is where all my programs work.

I have another iMac, late 2011. With an external SSD this machine is able to boot (Everything is on said SSD, no HDD inside). When I plug in this SSD, and try to boot, my iMac Retina 5K, 27-inch, 2017, I get the prohibitory symbol.

Does anyone know how I can fix this? I want to clone the external SSD into my new iMacs second internal SSD with 1TB, but this obviously does not work, since the external SSD is not bootable.

iMac Line (2012 and Later)

Posted on Mar 13, 2020 5:10 AM

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103,437 points

Mar 13, 2020 9:34 PM in response to JonasKloch

Hello,

I am sorry, since you performed this upgrade yourself there is not much we can do to help you on here. All you can do at this point is to fully erase the SSD and re-install macOS from a prepared USB installer.

How to reinstall macOS from macOS Recovery - Apple Support

How to create a bootable installer for macOS - Apple Support

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iMac trouble booting from SSD

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