Two days and countless hours later I came across a workable solution that in hind sight makes sense when setting up Premiere Pro CS 5.5 and CS 6 with a scratch disk on your Macbook Pro or Apple computer. If the standard route doesn’t work and you’re continually getting a read/write error, you may want to try this . . .
1.) First make sure your scratch disk is writable. Right click on the external drive desktop icon. Under the drop down menu choose Get Info. At the bottom unlock the spotlight screen. You will be prompted to enter your password. Back in the menu next to your Name and under Priveldge choose Read & Write. Lock and close.
2.) Open Premiere Pro CS 5.5 or CS 6. Click on New Project. In General, under location select your external drive under Browse. Next Name your project. At this time, don’t worry about the Scratch Disk settings. You will probably get a “scratch disk is write protected or unavailable” error or something similar.
3.) For the moment click through the chosen Premiere Pro disk location on your computer and open your new project. In Premiere Pro at the top of the screen open Preferences. Under Preferences choose Media. Under Media Cache Files choose Browse on your external drive and select a location for the folder. Under Media Cache Database select Clean. Next select Browse and select a location for the folder. Select Ok at the bottom of the screen and it will close.
4.) Next at the top of the screen select Project. Under the Project Settings in the drop down menu choose Scratch Disks. From here there are two ways to make the adjustment. One way is to select Project from the drop down menu. This should point to where your project file is being stored on the external hard drive. The second is to select the Browse button and find a location for each file on your external drive. Select Ok at the bottom and the screen will close.
It may take a little work but your scratch disk is now setup on your external drive. If you change drives at any time, you will have to go through the whole process all over again.
If this works for you drop me a note. Maybe someone at Adobe will hear that this is a major problem for those using their editing software on Apple computers.