If it's a Mac, then the operating system is almost certainly Mac OS X, if you don't know otherwise. (It doesn't really matter which version of Mac OS X it is, because things like how to format a drive don't change often.)
The issue isn't that the external drive is more complicated to partition and format. It's not. Partitioning and formatting a USB Flash drive would use exactly the same procedure as an external USB hard drive.
The issue, rather, is that most USB Flash drives come already partitioned into one big partition (which is probably what you want, because Flash drives have relatively small capacity) and formatted with a FAT32 filesystem (which is likely what you want if you need to be able to use them with Windows computers; otherwise, it may NOT be what you want -- more than once I've reformatted a USB Flash drive for ext2 or ext3, so that it can support things like symlinks).
Hard drives -- both internal and external -- usually do NOT come already formatted, because it's much harder for the manufacturer to guess what kind of partitioning and filesystem layout you would want. This is mostly because hard drives are so much larger than Flash drives, so it's far more likely that you're going to split them up a bit. For example, it's very common for a hard drive to have a "small" partition (which may still be larger than the entire capacity of the USB Flash drive) formatted FAT32, plus a much larger partition formatted with some other filesystem (for a Mac user, this might likely be HFS+ or possibly UFS; for a Windows user it would probably be NTFS, and for a Linux user there are a number of possibilities).
So yes, you do need to partition and format your new hard drive.
I'm afraid I don't know how to do that on Mac OS X, though. Well, I know a way that would probably work (using fdisk and mkfs from Terminal -- assuming you can figure out what your system is calling the device), but that's not the normal user-friendly way most Mac users would use. I'm sure there's something in the graphical user interface for doing this; I just don't know what it is, because I have only very minimal experience with OS X, and I've never used it to format a new disc.
If nobody can answer here, you could try asking on a Mac-oriented forum.