I have never really understood th best way to set up scratch disks.
Can someone please help me? I am shooting and editing AVCHD HD Footage.

I will provide the hardware I have if someone could please tell me the optimal way to set it up:

MacBook Pro with 128GB SSD, 6GB RAM
LaCie 2TB External Drive, 7200rpm Thunderbolt
Lacie 1TB Extneral Drive, 5400rpm Thunderbolt
Wester Digital Extenal Drive 1TB, 54000 USB 3.0


With this equipment: How should I set up the following for each project? The "?" represents what I am thinking
MacBook Pro: Program and OS?
Lacie 2 TB: HD Clips?
Scratch Disks:
Captured Video: Lacie 2TB?
Captured Audio: Lacie 2TB?
Video Previews: Lacie 1TB 5400RPM?
Audio Preview: Lacie 1 TB 5400RPM?

A common way to set up video project & editing storage is:
1) The editing software (Premiere Pro) is installed on the system drive- your SSD
2) All of the raw video files go on a dedicated "media drive"- probably the 2 TB 7200rpm since it is the biggest & fastest
3) I usually put the project file, auto save files, etc. on a third drive- maybe the 1TB Lacie Thunderbolt.
4) I also use this third drive as an "Export" drive for final renders of the project- i.e. rendering to Blu Ray, the file would be exported to this drive.
4) For scratch disks I would use "same as project" which would put them all on the 1 TB Lacie Thunderbolt.
Since you are shooting file based AVCHD rather than tape, there is not likely to even be any "captured" audio or video. Preview files should play fine from either the "Project/Export" drive or the "Media" drive.
The default for scratch disks when you set up a new project is "same as project", so I usually just go with it.
There are lots of variations on this theme, but these are some basic ideas.

I stick everything except renders on a RAID 1 2 TB drives from OWC. It has both USB 3.0 and eSata and the newer ones I think include TB. I've not had any problems with performance. I render out to a FW800 drive. I don't have any reason to change that. I find that I create a folder structure that has \yeardate_projectname\ and then a series of folders under that for:
Original Footage (or raw footage)
Renders (when I'm done)
Audio/Sound
Stills
Graphics
ProjDocs

I also create under the overall client heading (each client has a RAID array when they reach a certain size), a folder for special shoots that are used across multiple projects. So a full day shoot will get a special folder like
\2013_projectFoo\Live Concert at Foobar

That way, when the project is complete everything is in the same folder and I can archive it. That will also stay with the RAID array for the client.

Having been doing one client with numerous projects over the last few years, I can tell you, you want it as logically oriented as possible. You will need to find stuff again in a few years.

I also use a spreadsheet to track all shots. I eat the cost of doing that, but trying to find one song for example that was performed two years ago, when we were doing all day shoots at the rate of one a month, is saving me a lot of time looking for things now. I'm hoping Adobe will eventually be able to export their media management XML data into something resembling a tracking database. Right now, it's pretty much useless to me.