On using Disk Utility (DU) for changing the formatting type:

First: the obvious – changing the formatting type will totally erase the target disk, causing you to permanently lose everything on it (unless you have a backup, of course.)

Next: there are drives (the physical hard disk) upon which reside one or more volumes. When you see an icon on your desktop, you’re looking at the volume. The finder does not have a way of working directly with drives. That’s what Disk Utility (DU) is for.

When you open DU, you’ll see two panels. On the left is the listing of the drives and volumes that DU can recognize.

You’ll see something like:

465.8 GB ST3500630AS MacintoshHD

The uppermost one begins with the size of the hard disk, and its model number. This one is the drive itself Underneath that, and indented slightly, is usually one or more volume names. If you do not see anything underneath a disk, then it is not formatted yet (and you need to format it.)

You select a drive or a volume to work on in the larger right hand panel, by highlighting it in the left hand panel.

If you select a volume, then above the right hand panel you’ll see: First Aid / Erase / RAID / Restore

If you select a drive, then above the right hand panel you’ll see: First Aid / Erase / Partition/ RAID / Restore (note the addition of “partition”)

To select the formatting of a drive, first select a drive in the left hand column.

Next, click the “partition” button above the right hand column

Now in the right hand column, you’ll see

Volume Scheme: Volume Information:

Beneath Volume scheme, you’ll see a popup menu, probably saying “current” and beneath that, a diagrammatic representation of all the space on the drive.

Below that, you’ll see + – Options…

These are likely grayed out.

click on the popup menu directly underneath the words “Volume Scheme”, and select the number of partitions, (volumes) you want on the drive. “1 Partition” is the most common, although you may choose more.

Once you do that (choosing anything other than “Current”) the “Options…” button at the bottom of the diagram will become active.

Click on it, and select the format you want for the drive: GUID Partition Table; Apple Partition Map; Master Boot Record.

Then Click OK.

Click to confirm that you want to go ahead, and the drive will be reformatted in the selection method.

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And… only so it’s all in one place, here the bit from my previous post about those different formats:

And in that vein, here’s a bit about GUID (aka GPT) vs APM.

[GPT = GUID Partition Table = Globally Unique IDentifier Partition Table] [APM = Apple Partition Map]

When you buy a bare drive, it’s formatted as… none of the above: it’s MBR – Master Boot Record – which is what PC’s use, and why you need to format it with one of the others. (MBR is recognized by the Mac OS, but its structure imposes limits on what can be stored on the disk.)

You need APM to boot a PPC Mac, or use on a PPC Mac that isn’t running at least 10.4.8.

If you’re on an Intel mac, there is no reason at all to use APM (unless, of course, you’re planning on using the drive on a machine that meets the above criteria.)

And… if you’re on a PPC machine, and are running 10.4.8 or later, then the only drive you need formatted as APM is the boot drive (&, of course, any drives you’re cloning to with the intent of doing a drop in replacement.) The rest of your drives can be GUID, which offers some advantages.

Entirely anecdotally, the seem a smidgen faster to me… although it seems it’s somewhere in the 8-10% range (and I could simple be delerious, too.)

That said, they employ checksums on the partition map and header, as well as duplicates of each of those and the partition maps are larger. There are other tidbits which, like those I just mentioned, are not anything you’d ever notice in use… but make geeks like me really happy.

(OK… it makes your drive a bit more resistant to some kinds of corruption.)

But (since block copy was mentioned here recently) one needs to be sure that you’re using a block-copy program which is written for the GUID formatting, since programs only “familiar” with APM may yield a problematic copy (depending on its intended use). See Apple TN2166.

Upshot?

Intel: GUID all the way. PPC: OS less than 10.4.8 – APM all the way PPC: OS 10.4.8 > : APM for boot; GUID for everything else.