External Drive Partition, Lost Files

As the title suggests, I made a mistake that "erased" all my files on a backup external hard drive, after splitting a partition in two and installing Mac OS X Leopard on the new partition. I use quotes in "erased" because the data itself is still there, but the access to it isn't anymore. Before thinking about using recovery software and going through the harsh process of churning through hundreds of GB of data to recover files that could be corrupted in the end, I wanted to seek the opinions of more knowledgeable people to see if there are alternative solutions that could be more simple.

Here's my situation and all the steps that lead to my current state. I have a iMac with Core 2 Duo Intel processor, 2.66 GHz, 2 GB of RAM and an internal hard drive of 320 GB, with Mac OS X Leopard (10.5.8) installed. For backup and temporary transfer of files, I use an external USB hard drive, a Seagate FreeAgent with 500 GB (500 billion bytes = 465 GB actually).

I wanted to boot from my external hard drive to do some maintenance on my iMac's hard drive. So I used Disk Utility to split the current partition (the only one) on my external drive. The drive had 465 GB, give or take, with approximately 50GB free. The other partition to be created was 10GB. I know this is the kind of delicate operation that should be done with caution, but Disk Utility was saying explicitly that "this partition will not be erased", refering to the big partition that will become approximately 455 GB, the one that contains all my files. I say to myself, okay, that is safe.

Now, here comes the thing that I'm not 100% sure about. At this point, I don't know if the big partition was already erased. Both partitions (455GB and 10GB) always mounted on the desktop just fine. I was probably just "presuming" that everything went fine.

Then I proceeded to install Mac OS X Leopard from the installation DVD on the small 10GB partition, which went fine. It's only at this moment that I realised that my big partition was empty. Shortly after, I set the permissions of the big partition to read-only, assuming it would minimize further damage.

That's about it. At this point, my first thought was, what exactly just happened? In my mind, playing with one partition should not affect the other ones, but apparently, something did anyway, for a reason that is beyond my knowledge. Right now, I'm still wondering what operation did do the damage, the partition split or the installation of Mac OS X? Why would any of these operations erase the big partition?

And here's another important point to ponder. Before the partition split, my external hard drive was formatted as APM, Apple Partition Map. After the split, or after the Mac OS X installation (which one, I don't know), the partition scheme had changed to GUID. Maybe this is the key to solve the problem.

I know that my data is still intact, at least in most part, and I know that the file structure (B-Tree information, and stuff like that) seems to be intact (completely or partially) as well, because I visualized some sectors of my drive with a tool called iBored, and I recognized some folder and file names.

That's all the details I can provide. Right now, I feel like any low-level operation on my external drive is like defusing a bomb. One little mistake, and boom! So I prefer to do nothing until I can get some advice from others.

Thank you for your help!