Thanks!
avoid USB
As a sidebar, some folks may not realize that USB2 -- despite its specs to the contrary -- is slower than FW400 due to its greater overhead. And generally speaking, if you wish to have an external drive that is bootable (a useful option should your internal startup drive bite the Big One), you need FW400. In the long run, any saving from purchasing a USB2-only drive is not worth it.
So, yes, avoid USB.
Neil
Is it now okay to do this? If not, what happens if you do?
The problem that you may run into if you select External Drives for Scratch would arise if you later disconnect those Drives and Photoshop posts a "Disk not Found" error message.