I recently ran into this issue while helping a friend who was attempting to try Linux, (Xubuntu LTS Live USB.)
He was using a USB stick that was previously formatted to ntfs by Windows.
The USB loader saw a valid file system and simply installed the iso image on the stick.
It would boot to the try or install screen, but could not get past loading the image.
If you receive an error similar to this:
kernel panic . . . not syncing: VFS: Unable to mount root fs on unknown block (2.0)
Check your USB stick. If it is formatted to ntfs, erase it and re-install the iso using fat32.
The problem is that the live image is expecting fat32 and cannot load the system if the USB stick is formatted to ntfs.