1. Copy ALL data that you want to save, back on to the internal HD (ALL data currently on the external will be deleted during the next 9 steps!)
2. Open Disk Utility. (Found in the Applications > Utilities folder
3. Find your drive in the left hand column of Disk Utility.
4. Select it by SINGLE clicking on it.
5. Choose Partition from the Tabs in the Right Hand window. (RH window now changes)
6. In the dropdown menu, above the visual representation of the drive, choose 1 partition. (The dropdown probably says something like "Current" to begin with)
7. On the right of the RH window, you should see the word "Untitled" as the name of the drive. Change it to whatever your heart desires.
8. In the same area, you will see the format type choices. It probably defaulted to OS X Extended(journaled) or HFS+(Journaled) or similar. If it didn't, use the dropdown menu to choose that format.
9. Click the Apply button at the bottom left of the window. (It may say Partition or OK or something similar)
10. Let the Disk Utility do its thing. The drive will unmount from the desktop and then reappear using the name you gave it in step 7
That's it, you are done. The whole process, 1 through 10 should not take more than 10 minutes to complete.
Drives formatted as FAT are notoriously unstable and there is a distinct possiblity that you will wake up to an empty drive one day.
It is always best to format the drive for the Operating system you are using. NTFS for Windows and HFS+(Journaled) for Mac.
You may even find that all these instructions were in the box that this drive was shipped in. They are in the Seagate external drives.
Remember, ALL data, currently on the external drive, will be DELETED during this process.
Holler if you get stuck