Hold on a second, the seagate tool should let you copy a partition over and resize it at the same time. If it doesn't try to "download" or "borrow" a copy of PowerQuest Partition Magic, that's what I use everytime when I relocate my partition to a new drive and re-size a partition as needed.
How did it get so complicated? If you have 3 drives all you should do is:
1) Boot with your new drive (say 160GB) as master, attach a blank (say 100GB) as slave.
2) Blank the 100GB and create a partition on it with 100GB.
3) Copy all your data (family photos, videos, tax returns, love letters, etc, not programs or windows) from the new drive (160GB) to the 100GB partition. I assume it is not enough storage to put all your precious data.
4) Boot with your old drive (say 120GB) as master, attach the blank (100GB) as slave.
5) Copy as much data from the blank (100GB) drive to the 120GB drive as you can without overflowing it. Repeat step 1 to 5 if you cannot fit all your precious data from your new 160GB drive. Your goal is to unload all the data off the 160GB before wiping it out.
6a) Follow this step if you use the seagate tool to copy.
Now your old drive (120GB) and blank drive (100GB) should have all the important data from the new drive (160GB). So boot with the seagate software and re-partition the new drive (160GB) with a continuous partition of full size (160GB), and copy the content of the old drive's partition over (from 120GB to 160GB).
6b) Follow this step if you use partition magic (I would prefer this).
Boot with the seagate tool and blank or delete the new Drive (160GB). Then boot with old hard drive (120GB) and use partition magic to first copy the 120GB partition over.
7) Before you reboot, make sure you disconnect the old drive!!!!!!!!!!!!!!!!!!!!!!!!!
or else your new drive is going to be drive D, E, F, G or H instead of C as expected.
8) If you use partition magic, after booting with the new drive, use partition magic to resize the partition to full. If you use the seagate tool, your partition size should already be full (if not, then it is not supported and you should use partition magic).
9) attach the blank drive (100GB) as slave and copy over all the data you wanted to the new drive.