I have a laptop that boots up into Windows 7.
I often run Ubuntu inside a virtual machine on that laptop.
There are two key things that need to be downloaded and installed to get Ubuntu running inside a virtual machine:
- Download the Virtualbox application ".exe" installer into your Windows Downloads folder. Then install the VirtualBox application (or some other virtual machine) on the host OS -- in your and my case, we're installing "Virtualbox for Windows hosts" on a Window 7 host OS. Wikibooks: VirtualBox / Installation / Windows has more details.
- Download the Ubuntu ".iso" image file into your Windows Downloads folder. Then run VirtualBox, tell it to create a new virtual machine and boot that virtual machine from that ".iso" image, and inside that virtual machine install Ubuntu. Wikibooks: VirtualBox/Setting up an Ubuntu virtual machine has step-by-step instructions.
After you get Ubuntu installed and rebooted a few times, you can delete those downloaded files from your Windows Downloads folder.
I never needed a USB flash drive during the initial installation or while running Ubuntu as the guest operating system inside a virtual machine.
Honestly, I have no idea what you mean by "I used LinuxLive to install VirtualBox on a 16 GB flash drive."
I'm assuming there are a bunch of typos in that sentence.
(I've seen people who wanted to completely erase the hard drive of some computer and install Linux on it. Making a "LiveUSB" bootable USB flash drive so they can "Try Ubuntu" is a good first step in that direction, but it sounds like that's not what you want to do).
(I've seen people download the "VirtualBox for Ubuntu hosts", so they can run some other operating system inside a virtual machine on their Ubuntu host computer, but it sounds like that's not what you want to do).
Normally I see my normal Windows taskbar on the screen, and a big window with a title bar that shows the name of that virtual machine followed by "- Oracle VM VirtualBox". I run all kinds of Linux apps inside that window.
Next to that big window, I see a separate little window with a title bar that shows "Oracle VM VirtualBox Manager".
In that little window, I click on the name of the virtual machine, then I click on "Settings", then I click on Storage" to get the Storage Tree view.
If you accepted all the defaults (such as by following the above step-by-step instructions), by default there's only one "disk" in that storage tree, and when I click on it I see that it is actually a file located in my "C:\Users\dc\VirtualBox VMs\" folder.
That file is what the Ubuntu OS inside my virtual machine sees as the "/dev/sda" hard drive.
So what is /dev/sda?
When running Ubuntu, "/dev/sda" is what Ubuntu thinks is the computer's hard drive.
When I see people boot Ubuntu from their hard drive so it fills the entire screen (i.e., Ubuntu is actually running directly on the physical hardware), "/dev/sda" is the actual physical hard drive.
When I see people boot Ubuntu from a live USB flash drive so it fills the entire screen (i.e., Ubuntu is actually running directly on the physical hardware), they see both a "/dev/sda" and a "/dev/sdb".
I can never remember which one is the actual physical hard drive and which one is the actual physical live USB flash drive.
(Asking how you can tell one from the other would make an excellent separate question).
When Ubuntu is running inside a "...- Oracle VM VirtualBox" window, "/dev/sda" inside that window is whatever the virtual machine is using to simulate a virtual hard drive. What Ubuntu sees as "/dev/sda", when that Ubuntu is running inside a virtual machine on a Windows host, is usually a ".vdi" file in some folder on the C:\ hard drive somewhere. I hear that occasionally some people change those settings so that what Ubuntu sees as "/dev/sda", VirtualBox can remap to some physical hard drive or USB stick rather than the default ".vdi" file. You can find out exactly what that is on your system by checking the settings in the "Oracle VM VirtualBox Manager".