Q: Some tips to get MAC moving faster.

Which iMac do you own (what year and model) and how much RAM is installed?

Also, please let us know the kind of EHD you use, how it is formatted and how it is connected. Connections can be USB, Firewire, etc...

To locate how to find out how much RAM is installed click the Apple symbol in teh upper left of the display and then click About this Mac and it will tell you how much RAM is installed.

If in fact 4GB is what you have installed, then I'd probably recommend a RAM upgrade. However to be sure I would strongly recommend reading Apples advice letter that discusses an app you have on your computer called Activity Monitor so you know how to use it. This will tell you how to determine if your computer will benefit from a RAM upgrade. The article is called Using Activity Monitor to read System Memory and determine how much RAM is being used (OS X Mountain Lion and earlier) and you can find it by clicking:

http://support.apple.com/kb/ht1342

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.

I have an external drive to save stuff on--seems like since I have used that, thats when all this started.

Disconnect it and see whether there's an improvement.