MY iMAC IS RUNNING LIKE A SNAIL

I have been a Mac user for about 4 years now. I own a Mac Pro that I use for my Photography Business and several iMacs for other purposes, One for Amateur Radio Communications and one for Engineering Purposes and some Photography. One of my iMacs is running very slowly. It has always been extremely fast this one I also use for personal Photography... but, about a month ago I noticed it was taking very long to upload RAW photo images and when I load these RAW images into Photoshop CS4 or Lightroom it takes forever to load them. I have run some of the Disk management and test routines but I cannot figure out what is wrong. My other machines seem to be fine. This machine consists of the following.

OS X V 10.6.8
Intel Core 2 Duo 3.06 GHz
1.1GHz Bus Speed
Memory 4 GB RAM
500GB Internal HD
1-TB WD External USB Interfaced Hard Drive

Suggestions & Advice are most welcome.

Cordially,


Don

Not sure what you have tried so far but try these:
Run Disk Repair and Repair Permissions from Disk Utility
Download and run the automated scripts in Onyx for Snow Leopard
Reset your PRam Resetting your Mac's PRAM and NVRAM

Make sure you have free space on your internal Hard Disk - I'd suggest at least 20GB
Check to see that you don't have any unnecessary/memory hogs running on startup (System Preferences - Users Accounts - Login Items)

My reply 2 u

Thank you mrPlow for your prompt response and the information... I appreciate this.

Don