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: Note that will need a working Mac to download to. To force the date and trick the installer, take a look at this post:įailing that, or if you just want a clean installer, at the time of my reply there's a full El Capitan download here: get your updates (there will be a bunch).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: 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. 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.

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. What's happening is the computer date is past the certificate expiry, and it throws an error. This sounds a lot like the installer certificate has expired. I used to provide USB installers for old macOS, including El Capitan. Two options to try here: bodge the date, or get a new installer.

El capitan dmg bootable usb