Erasing personal data from old iMac before recycling

I want to recycle my old iMac. I had stuff on the desk top when i began this process. I used the disc utility and erased the free space (out of order I think). This took like 14h. The I went back in and erased the HD (another 14h) and then I went and erased the free space again. How do I know if all my personal files are gone. I can start the computer again only in recovery mode but can't install High Sierra (says can't reach server). I don't really care since I am going to recycle the computer. I just want to make sure all private info is gone. Is there an easy way to make sure everything is gone?

[Re-Titled by Moderator]