I am not talking about Fedora 20 LiveUSB but currently I am running the latest Fedora 20 Gnome edition on my USB flash disk. I was install Fedora 20 to my USB flash disk without problems. Installing Fedora 20 to an USB flash disk is almost the same as installing it to hard disk. The main difference is that we replace the hard disk with the USB flash disk. There are some important things we need to remember when trying to install Fedora 20 to a flash disk.
1. Enough space
I use a 16 GB Transcend USB flash disk to handle the installation. But I think, a 4 GB flash disk will also works.
2. Do not mount the flash disk
If you want to install Fedora to USB flash disk, you have to make sure that the disk is not mounted. Or, simply unmount it first before starting the installation process.
So there is another solution for you who want to try Fedora 20 without having to install it on your hard disk. Try to install Fedora on USB drive.