Does Mac OS X use external drives for scratch space?

No. It's not needed with OSX, as the OSX does defrag periodically (unlike a certain poorly made os).

You can use other disks for scratch disk for media apps though.

 

OSX must use some of the primary disk as swap... How else does it save the RAM to the HDD when it goes to sleep?

But no, I don't think any sane OS would use USB-type external HDDs for swap. It can too easily accidentally be disconnected and that would be problematic.

 

All computers use the system drive as a scratch disk, to not use the system drive would defeat the purpose of "scratch disk"

 

If you get OS X running on ZFS it would, but why would you want your scratch disk to be external, let alone USB???

 

If you are comfortable with mucking around the OS (e.g., at least intermediate+ level Unix knowledge) you can move the swap space to another partition. See

http://forums.macosxhints.com/showthread.php?t=81586

to get started. There are other resources too, if you want to search for moving the swap.

It is too much trouble for 'normal' users and the OS manages swap just fine, so it is better to leave it alone unless you have some very specific need.