There is/was a program(paid) that would partition the drive without destroying the contents. I believe it was/is called iPartition. Google for it.
You can usually install software on any drive that is attached to your Mac but bear in mind that it is not just the application that gets installed. While the app is located someplace, the preferences, library entry, application support data(if any) will be installed on the drive from which the machine is booted.
When you move an application from the internal to the external, it will continue to run correctly. However, when you delete the contents of the main drive, you may well be deleting an integral part of the application, one that is required to make the application function correctly.
Probably the best way to move everything from one drive to the other is to use an application like Carbon Copy Cloner, there are others, to make an exact copy of your internal and then transfer it to the external. The external now become bootable and you are free to mess with the internal.
Another way to go would be to leave the internal drive exactly like it is, with whatever OS version is currently on it, attach the external FIREWIRE (preferably Firewire 800 if your Mac supports it) connected hard drive (go for 500GB or more) and then install the new OS onto it. That will save you the hassle of trying to ensure that all your existing programs continue to work correctly if you move the.
You then have the choice of which OS to boot into when you start up your Mac. Remember that you cannot have 10.4 and 10.5 running at the same time.
It would have helped a bit if you had mentioned exactly what sort of Mac you had.
Do you have a particular reason for wanting 10.4 and 10.5 on your machine?
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, 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?