The two best options are probably to start over or move one of the large directory structures like /usr or /usr/bin over to the new drive, then mounting it in place of the old directory.
Unfortunately, it's not quite as simple as copying the files. Almost, but not quite.
Check this page out for an example of how to move your /home directory: http://www-106.ibm.com/developerwork...-partplan.html
I think I have another example around somewhere too; I'll post it if I can find it.
BTW, one of the problems is that you need to delete the items under /usr or /usr/bin on the old drive before you mount the new one, but the commands for removing the files and mounting the partition are there, so it can get tricky.