Thanks for responding, my Mac stopped booting up (gray folder with question mark) so I bought a replacement drive and booted up using El Capitan Install USB to install onto the new drive, but this error popped up at around 90% during the installation.
In my spare time I tested the old drive on another mac via SATA-USB cable and it works with no problem.
Now I suspect the SATA cable was and is at fault, but I'd rather be sure before blowing my money again on the wrong assumptions. Please help, thanks!