Reboot the Mac with the external drive physically disconnected
Run Disk Utility.app from /Applications/Utilities/
Connect the drive. Now this might be where things are getting mixed up. Disk Utility will show at least two entries for the drive. The top one is the physical disk itself. The lower one (or more than one) are the partitions on that disk. (See the image for what mine looks like.)
Click the partition (#1) and then look down where it says “Format” (#2). If it isn’t Mac OS Extended (Journaled) then try to unmount it (#3). Repeat this step for every partition (if there’s more than one).
Reformat The Drive
Once the partition(s) is/are unmounted, select the main listing for the drive (#1)
Then select the “Erase” tab (#2)
Make sure the Format is set to Mac OS Extended (Journaled) (#3)
Give the drive whatever name you want to use for it (#4)
Click “Erase” (#5)
That should bring up a dire-sounding warning that you are about to delete everything on the drive. That’s unavoidable when reformatting, so make sure there isn’t anything you want on there!
If that doesn’t do it, I’m not sure what else to suggest other than bringing the drive back. You can probably order a Mac-formatted one from Amazon.com.