Add El Capitan to a Partition on an SSD

Is this an old El Capitan installer you had lying around. In October the certificates for many mac OS's

expired, so these older installers will return that error. If you are trying to install El Capitan via macOS Recovery then I don't think Apple have bothered updating the certificates on those OS's either.

Apple did however release an updated version of El Capitan at the end of October last year, download it from here, How to upgrade to OS X El Capitan – Apple Support go to section 4 and follow the instructions on how to download the .dmg, open it to the .pkg, and then convert it to the Install OS X El Capitan.app.

Otherwise the date and time change should work as mentioned before.

Go offline and then to the system preferences and change the computer date to a few weeks past the date the El Capitan install package was made. It's a signing issue inside the install package itself, so the system won't accept older installers. Rubbish, I know - dealt with the same thing recently. Let it reboot after the install and then when it is up and running, go in and get back online and it will correct the date and finish the update packages.

Good luck!

Edit: FYI, the open-source office suite called LibreOffice opens Claris/AppleWorks .cwk files. Even maintains 99.98% of the formatting, the only iffy bit is some of the text kerning, but you can fix that easily enough by changing the font..

Tell me about your internal disk and its partitions. Do you want to install El Capitan over your Snow Leopard installation, or do you want to install El Capitan onto a new partition on your internal disk.

If you have partitioned your disk to two partitions how did you do it and are they Mac OS X Extended (Journaled) on a GUID Partition Map.