Stick with FW800 external drives. No, there's no more Scratch Disk (thank goodness). Inside FCP X, you create Events on any hard drive attached. You create Projects on any hard drive attached. You can even use Disk Images, too. The user manual explains all of this very clearly.
I'd stay away from USB for video editing all together. Thunderbolt will help with really large projects, and work using very high end, very large HD and digital film files.