When I moved my OS (X Snow Leopard) and my files to a bigger disk this week, I used Apple's wonderful Migration Assistant (which offers itself automatically when you upgrade to a new Mac or disk). This is a brilliant app which has taken upgrading from being a long and complex business to being pretty much a one-click affair. When you start up on the new system/disk/computer, everything is like before, except for more space/speed/features. Outstanding.
There's just one thing: if you have a lot of files (like around 800 GB in my case), it takes many hours to do this, during which you can't use the computer at all. This I don't care for, so I got tricky: I took the two heaviest folders, both containing graphics, and moved them outside my user folder on the original disk. And then I deselected the "other files" option when using Migrating Assistant.
Well, so far so good. I was up and running after a couple hours instead of sixteen or whatever it would have been. Now I could copy those big/many files at leisure while I continued working and playing.
There was just one but: the macro programs and utilities (like QuickKeys and FinderPop), if they referred to an object on disk, they smartly now refer to the same object on the new disk, as is right. (I think it's the OS arranging it, not the apps.) But in those cases where I had not yet moved those objects, it couldn't find them, and referred to the object on the old disk instead!
I'm not sure if this is really clever, or... hmmm. I think I would have preferred to know about it. I remember last year, Photoshop has used a preference file on an old disk, which I did not find out until I removed the disk and lost my preferences.
Anyway, when I found out about this (I moved a file via FinderPop and then couldn't find it), what I did was that I ejected the old disk (in software). This way I get an error message when some utility wants to refer to an object on it, and I can fix it.