Can you install OS X from a USB on late 2010 iMac? I'm having quite a few difficulties with my old, late 2010 iMac, and upgrading from Lion to Siearra did not help. I have used my Laptop to create a USB Boot for El Capitan, and I would like to use it to downgrade my Desktop to El Capitan.

I know these older models of iMac cannot install Windows, via Bootcamp, from a USB, but is this the same for OSX? Any help with downgrading/fresh installing to El Capitan would be much appreciated.

P.S. My iMac's super drive is broken, so I cannot install anything from a disc. It's USB or nothing, I'm afraid.
Your iMac late 2010 is compatible with Sierra according to the official MacOS Sierra - Technical Specifications page.


  Mac Hardware Requirements
  
  For details about your Mac model, click the Apple icon at the top left
  of your screen, choose About This Mac, then choose More Info. These
  Mac models are compatible with macOS Sierra:
  
  
  MacBook (Late 2009 or newer)  
  MacBook Pro (Mid 2010 or newer) 
  MacBook Air (Late 2010 or newer)
  Mac mini (Mid 2010 or newer)
  iMac (Late 2009 or newer)
  Mac Pro (Mid 2010 or newer)
  


You can still get the OS X El Capitan via this link - thanks to @user3439894 for pointing out. It's up to you what OS installation to proceed with, but as long as your hardware supports the newest OS version I would go with clean install of Sierra.

Before proceeding further make sure you have your data backup somewhere, preferably using Time Machine. Also make sure you have a working memory stick with 8GB or more on it.

You can create a bootable USB drive with macOS Sierra on it following these steps:


Go to App Store and Download macOS Sierra. You can do that even you already have the macOS installed.

When the download will be finished the popup will appear asking you to proceed with installation. You need to close it. You can double check the location of your Install macOS Sierra.app in Applications folder.

With your memory sticked inserted in your you need to open Terminal (/Applications/Utilities/Terminal.app) window. And copy/paste the following command:


sudo /Applications/Install\ macOS\Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app

For El Capitan you'll have to type this command:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

Where Untitled is the name of your memory stick.




Then you need to type Y acknowledging the fact that your memory stick will be erased in order to create bootable media.

Copying installer files to disk process can take some time depending on type of your memory stick (2.0 or 3.0) and your internal HD (HDD or SSD).
When the process is finished you will see the following screen:

You can proceed with restarting your iMac keeping the memory stick inserted.
You need to hold ⌥ key during the startup in order to go into Startup Manager. 

You need to choose Install macOS Sierra from the available options.
Go to Disk utility and erase (OS X Extended (Journaled)) the current startup drive - the one you're running your OS on. This will format your drive and delete everything on it.
When the formatting process is finished, close Disk Utility and select Install macOS from the menu. Make sure you're installing your macOS on the startup drive not your memory stick.


Following these steps will help you with your macOS clean install. If you feel you get stuck somewhere you can always consult with this guide or this one. There are many other guides available.