With SharePoint 2013, Upgrade Evaluation Site Collections can come in quite handy. They allow you to test what a site will look like once upgraded without affecting the site you just attached. If there are issues, you can just correct them in the source 2010 version site, and run the upgrade process again. You can request an upgrade evaluation site collection, by clicking Try a Demo Upgrade after clicking on the bar at the top of your site that says Explore all that SharePoint 2013 has to offer.
If you have a rather large site collection though (greater than 100 GB), you will find that you get an error when you click the button to upgrade.
Cannot create upgrade evaluation site for site collection http://server/sitecollection because it is larger than 100 GB. The request must be made by a farm administrator.
Wait a minute though! I am a farm administrator! How odd? What to do? In order to get around this, I decided to go with PowerShell. In my talk SPC195 from SPC12 last year, I touched on how to request an upgrade evaluation site collection using PowerShell. I put those slides to work and executed the command below.
Running this cmdlet will return nothing if successful. It doesn’t actually start the upgrade evaluation site collection process though. By default, that executes using the Create Upgrade Evaluation Site Collections job nightly around 1:00 am. You can make it run now though by finding the appropriate job definition for the web application in question and running it now. Next, all you have to do is wait. After all, your site collection is fairly large so that means you are going to be waiting a while.