I've had the same problem the entire night. It's due to the installer not recognizing the USB drive after it's booted to BusyBox.
To fix this, you need to press F6 and modify the boot options. First, remove "quiet splash" from the options and add "break debug" in it's place. Continue installation as normal from this point. Eventually, it will break and you'll be presented with the BusyBox shell, and see plenty of the errors relating to this.
Are you ready for the next step? I promise you, it's an advanced manuver that even Han Solo himself would struggle to attempt.
Step 1: Unplug your USB drive
Step 2: Plug your USB drive back in
Step 3: Type exit and press <Enter>
Step 4: Wait 5-30 seconds and you'll be booted into your Ubuntu graphical installer.
Note: I've verified this works for Ubuntu and Linux Mint. I've been working on this problem for about 4 hours tonight and this is what worked for me, after trying every imaginable method of configuration flags for both the installer and BIOS. I am using a Toshiba Satellite C655D and using a 32GB USB 3.0 flash drive as the installation media.
I hope this helps someone, as I was about to go back to Windows after this nightmare.