I tried to perform a clean install for Mac Pro El Capitan. The app store said it was unavailable so I purchased a bootable usb installer but it won't install. What do I need to do
Two options to try here: bodge the date, or get a new installer.
I used to provide USB installers for old macOS, including El Capitan. This sounds a lot like the installer certificate has expired. What's happening is the computer date is past the certificate expiry, and it throws an error. Without knowing the exact error, it's hard to be 100% sure, but El Capitan was one of the more frequently twitchy installers in my time doing USBs.
The installers were re-signed in 2019 to last until 2029, but if you've bought a USB from someone, it's entirely possible it's a pre-2019 image and has the expiry issue.
My suggestion would be to reset the date on the computer to a time in the past when the certificate was valid, install El Capitan, and when you're up and running you can reset the date to the correct date. I don't think you'll have any issues with downloading software updates for it after you're installed, so it should be safe to:
- force the date to a time in the past
- install El Capitan
- fix the date to the correct one
- get your updates (there will be a bunch)
To force the date and trick the installer, take a look at this post:
Failing that, or if you just want a clean installer, at the time of my reply there's a full El Capitan download here:
Note that will need a working Mac to download to. You'll need to download it, open the DMG, run the installer inside it which will make the
Install MacOS El Capitan.app and then you need to make a USB, which you can do with this link:
The command for El Capitan is:
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
You'll need to change
MyVolume to the name of your USB installer, but it should take care of the rest for you.
To boot from your bootable USB, hold the alt or option key (depending on if your keyboard is Windows or Mac) on startup until you see options on what disk to boot from. Then, select your bootable USB and boot up from it and it will bring up an installer. If it shows a / with a circle when you click the USB installer, the Operating System is not compatible with your computer.