Reformatting a new external hard disk? You wouldn't think this would constitute any challenge, would you? You start up Disk Utility, you select the external disk, you switch to the Erase panel, and you take a deep breath and click Erase, right? Wrong.
It happens that, perhaps because of the changing economics of external hard disk acquisition (fancy talk for "they've gotten a lot cheaper lately"), I've recently had to reformat several new external hard disks. These include a shirt-pocket sized Maxtor Mini for taking my compressed music collection along on airplane trips, a larger Maxtor OneTouch that I made a friend buy when I discovered that his wife's compulsive ripping of The Prisoner episodes from the local library had filled up her iMac's internal disk, an AcomData to serve as my mother's iMac's Time Machine backup, and most recently a whopping 1 TB Fantom GreenDrive, a rugged, cool, silent machine that I picked up for less than $100 at Buy.com. In every case I started by trying to use Disk Utility's Erase panel, and in every case I encountered some sort of initial failure. In the case of the Fantom drive, there were even printed instructions saying to do this, and they were wrong. Wrong, I tell you!
So, since experience has taught me the right way (repeatedly, because I so readily forget what experience has taught me), I'm going to give you the benefit thereof and put this canard to rest once and for all. This is what you do:
Launch Disk Utility. Plug the new external drive into your computer, provide it with power as needed, and switch it on. When the new disk appears in Disk Utility, select its top-level icon. (I stress this because the disk is represented by two icons, one for the physical disk, as it were, and one for the single volume it contains.) Now switch, not to Erase, but to Partition.
On the Partition pane, everything will appear to be greyed out, as if you had encountered a brick wall. That's because before you can do anything, you have to change the partition arrangement, using the Volume Scheme pop-up menu. You have to do this even if you don't actually want to change the number of partitions. So, the Volume Scheme pop-up menu starts out saying Current. Change that. The minimal change is to 1 Partition. I'm not going to tell you that you need any more partitions than one, or how big they should be; that's up to you, and depends on how this disk will be used.
Now stop. Stop! I know you think the next thing to do is give the drive a name and assign it a format - probably Mac OS Extended (Journaled), the default (and rightly so). But don't do it yet. See the Options button below the rectangular graphic depicting your partition scheme? Click it. Click it! This is the key, all-important step. From this one step stems all the trouble or goodness, the success or failure that your reformatting of this new external hard disk will be met with.
Why? Because there are three possible partition schemes, and many disks come with Master Boot Record, which is absolutely wrong for a Mac. You must choose between GUID Partition Table and Apple Partition Map. The latter is the most universal for use with Macs; you can't go wrong this way, unless you want to use the disk as a startup disk. If you do, then your choice here depends on what kind of Mac you want to start up from this disk. Intel-based Macs prefer GUID Partition Table; they can boot from disks partitioned using Apple Partition Map, but won't let you install Leopard to such disks directly (you must clone a copy of Leopard from a GUID-partitioned disk to get this to work), and will prevent you from installing firmware updates on your Mac while you're booted from such a disk. On the other hand, PowerPC-based Macs can boot only from an Apple Partition Map disk. (See Jonathan "Wolf" Rentzsch's "," 2006-01-30.) Apple also  that the same distinction applies if the disk is to be used as a Time Machine backup, though I'm not entirely certain why.
So choose your partition scheme and click OK. Now enter a volume name and a format, and click Apply. Presto! The disk is reformatted in the twinkling of an eye, and is ready for use.