Dumb question. My MBP 2010 has an installed-by-me Samsung Evo pro 500 mb boot drive. I tried to update from Snow Leopard to El Capitan the other day. The computer crashes shortly after boot-up.
Your problem is probably old files left on the computer from previous upgrades.
The same thing would have occurred if you went from Snow leopard to anything after it. 10.6 was pretty tolerant.
I am assuming that you of course made a TimeMachine backup of your computer before you upgraded it?
If you did then I suggest restoring it back before doing the next exercise.
Go through the Mac HD / Library folder and the User / Library folder and remove anything that is really old. This means old printers, old programmes, etc.
Use the Font Book app in the Applications folder to remove any broken and duplicate fonts.
If you then restart the upgrade process then stop after the download and make a boot USB stick. Google "How to make a bootable OS X installer drive"
This allows you to have something you can recover from without having to download the whole thing again. Good luck.