I have my external hard drive partitioned into 3 compartments (using Disk Utility).
1. Time Machine (Mac OS journaled +)
2. Carbon Copy cloner (Mac OS Journaled +)
3. Fat32 for my PC data
Fat32 format is readable and writable for both mac and PC. However, an individual file can't be bigger than 4 GB. It is nice if you want to go back and forth.
For your data (if all smaller than 4GB), format your external hard drive to fat32 (using disk utility) and copy your pc files over to your mac.
You could also reformat using NTFS (pc format) but it is only readable on a mac so of limited usage...but the files can be huge.
There are also software out there that let you write in NTFS on a mac...but they cost money.
I highly recommend you making a bootable backup. So if your hard drive on your mac dies all you have to do is hook up your portable and run it from there, no time lost.