Finally, we can start up our virtual machine and install SteamOS. Frankly, this process is a bit confusing and involves a fair amount of manual tweaking as you go. Follow along with each step, and you shouldn't have any problems getting into SteamOS.
Double-click on the SteamOS machine's icon in the left-side column of VirtualBox. A new window will open, which acts as the "monitor" for your virtual PC. After just a few seconds you'll see the SteamOS installation menu in it. (If you see "Error: Prefix not set," just wait it out.) Choose the first option, "Automated Install," using your keyboard. It says it will erase your disk, but don't worry! That's only true inside the virtual machine. Your actual hard drive will be fine, which is why we're doing this in a virtual machine to begin with.
After you click it, the system may seem to freeze for some time, then the SteamOS installer will begin, and automatically install all the files it needs. This process will take several minutes. When it's done, the machine will reboot, and briefly give you the option to start in recovery mode. If you don't, the virtual machine will simply go to a blank screen. If this happens, reset the virtual machine (Machine > Reset), and choose to start in recovery mode while the option is available to you.
You'll now see a command prompt, where you'll have to enter several commands. First type apt-get purge ".*nvidia.*" and hit enter. Some stuff will scroll by, and you'll be asked for a "y/n" confirmation. Type "y" and hit enter.
After more furious scrolling of text, you'll again find yourself at the command line. Enter dpkg-reconfigure xserver-xorg and press enter. If you don't get any confirmation at all, but just see another command prompt, that means it worked.
Now, we'll need to install VirtualBox guest additions — a suite of software that essentially allows the Debian-based SteamOS to function in a virtual environment. To do that, follow these steps:
First, click on the Devices menu at the top of VirtualBox, and then click on Insert Guest Additions CD Image. (You won't receive any confirmations that you've done so.) Next, type in mount /dev/cdrom/media/cdrom and hit Enter. (Note the space after "mount"!) The virtual machine will say something about mounting read-only, but that's fine.
Next, enter the command sh /media/cdrom/VBoxLinuxAdditions.run — once again, note the space after "sh" — and hit Enter again. Lines of text will scroll up for a few seconds as the virtual machine installs the Linux guest additions. When it's done, restart the computer by entering reboot and hitting Enter.
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