how to keep my software and partition hard drive along with an external hd

Hello,

This question might sound dumb but Im new to how External Hard drives work. So hopefully someone can ansewer a question for me:

If I set up an external hard drive for my mac, Can I put and run software on it?

The reason Im asking is because I would like to partition my mac with osx 10.4 and osx 10.5 and to do this I would have to reformat (erase to factory settings) my mac. If I do this I will lose all of my software.

So Im hoping if I use an external hard drive, I can copy all my software to it. Then partition my mac, and then send all my software back to my mac.

Does anyone know if something like this will work? or have any other suggestions?

I have a lot of third party software on my computer that I cant get again (I dont even know where half of the serial numbers are anymore whichis why I need to figure this out), they are all in my applications folder sooo I was hoping to copy the contents to the external HD then put it back.

Will the software still work?

Thank You,
Sincerely,
Michael

First, you'll want to clone your HDD to the external. Use SuperDuper or Carbon Copy Cloner. Both will make a bootable copy of your HDD.
It also depends on how you want to setup. You can partition the external and put both OS's on that. Or do as you mentioned, if you have the space.

Which Mac are you using? And which type of external HDD? (Firewire or USB)