Is it "cleaner" to start a migrate manually instead of with the assistant?

The external is Mac OS Extended (Journaled), yes.

I do see that in the Partition tab of disk utility, it says "To erase and partition the selected disk..." Is the only way to make a clone (or to partition an existing HD and put a clone in the new partition) to erase it?!?!

Well it's a good thing I'm getting this new computer because mine is being very weird and very slow. I can't attach the image to this message for some reason. You should be able to see it at http://www.tuzmusic.com/partitionscreen.png

Note, by the way, that I am on my current computer which is Lion, not Mountain Lion.

Oh, now it's working. Here is the picture.

Well now it works. Here it

Well I partitioned a different drive (once I hit "Apply" a message came up saying it wouldn't erase anything) and am currently creating a clone on this HD that I will use with Setup assistant.

So I guess back to my original question though: this original computer has been slow and a little weird (it's been acting up a lot since I started all this partitioning and duping and everything). It seems unlikely to me that this is all about its "physical body" being 4 1/2 years old and not about having a good amount of "junk" on it slowing it down. Which would mean that if I am transferring all the junk I currently have (minus some cleaning) through Setup assistant to a new computer, it's just the same junk on a newer computer. Am I totally wrong with this?

The actual practical question is, once I have the clone, what are the best places to clean up so that my new computer is clean as can be?

It is indexing so that sure makes sense.

In terms of cleaning up, that was indeed my point. What I want to know is what are the best ways to clean up? Not necessarily the tools to use, I mean, moreso what to look for to clean up and where to find it.

How do I boot from the clone, and why is that better than just cleaning up the clone as if it were a hard drive I was cleaning up?

So it sounds to me like you're saying the only thing that might be "polluting" and slowing down the computer, besides indexing, and besides the actual physical age and wear on any parts, is applications and their associated files. And that any other data -- documents, music, etc -- don't make a difference. Am I understanding this correctly?

Thanks folks, this is very helpful! I guess all these ideas about the computer being "polluted" goes back to Windows days. It definitely does seem like I will just "upgrade" from my current computer. If I wanted to run Setup Assistant to use the clone, would I have to do it by booting up my old computer from the clone? Or could I just hook up the clone's hard drive to the new computer, and keep using my old one while the new one migrates? If not, I'll just run Setup from my old one as you say.

So I'm looking at the login items and most I recognize (helpers for itunes, dropbox, etc). I wonder if anyone knows about any of these and whether I can get rid of them:

GrowlHelpeApp (I do know what Growl is, so I'm guessing this is what allows it to work)

OpenDNS Updater (I read an article about OpenDNS quite a while ago, and I installed it but I don't even know if I set it up in a way that actually runs.)

SIMBL Agent

GetBackupAgent

HPScheduler and HPEventHandler (We used to have an HP printer but don't anymore. Also I used to have a Palm Pre but I think that was even before they were bought by HP so I doubt this is related)

Also, at what amount of free disk space would that start to effect the performance of the computer?

Well thanks a lot MacJack and CSound, I think I'm all ready to migrate when my new computer comes later today!