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I have an iMac that I inherited from the previous IT director. I’ve searched all over Google and spoken with Apple support, but I cannot upgrade from Yosemite to El Capitan. I need to be on El Capitan so that I can use an app that is required for my job. I have a USB set to boot from, but the Mac will not see it, if I try to boot from it. Any help would be greatly appreciated.
OK lets get the installer from here {https://support.apple.com/en-us/HT211683|How to get old versions of macOS] format your USB thumb drive with GUID and a journaled file system (HFS+) and then follow this guide How to make a bootable OS X 10.11 El Capitan installer drive Then reboot using your USB drive
