Q. How to set up a new external hard drive on a Mac?

A. When you plug in a new external hard drive to a Mac, it will either be formatted for Windows (in which case you may have trouble writing to it from the Mac) or it will be unformatted. An internal hard drive replacement or upgrade is almost always unformatted. In order to use a drive, you will need to Partition and Format it for your machine.

• NOTE: KEEP IN MIND THAT ALL THESE FORMATTING AND PARTITIONING OPERATIONS WILL DELETE ALL DATA ON THE HARD DRIVE.  Make sure that you back up the data from the hard drive first, if it has been used.

When you first plug in an external drive, if the Mac doesn’t recognize the formatting, it will pop up a message “The drive cannot be used, do you want to intialize it?” (Initialize means the same as Format). You can go ahead and let it initialize, but I recommend that you learn how to partition and initialize a drive yourself.

Initializing (Formatting):  Formatting means preparing a drive for accepting data by organizing its sectors and creating catalogs – a drive must be formatted before it can be writtten to. The format method used on a drive depends on the operating system.  Windows machines can use NTFS (Microsoft’s proprietary filing system) or FAT-32 (the older system with roots back to MS-DOS days). Macintoshes use HFS+ (called Mac OS Extended in Disk Utility) and can also read and write FAT-32 (called MSDOS in Disk Utility) However, for a disk to be an OSX Boot volume, the format must be HFS+.
You can see right away that a drive or USB memory stick that is intended to be used between the Macintosh and the Windows environment should be formatted FAT-32 for compatibility.

• (As an aside, a few cameras and flash memory products use FAT-16 or ExFAT; the rule with camera and device flash memory cards is to always format them in the device, and never on a computer)

Partitioning a drive commonly means to divide it into two or more sections, each partition can be formatted independently, and each will mount on the Mac desktop as a separate volume. (A drive with a single volume of the whole size of the drive is still partitioned, it has just one partition).
In the illustration below, my 2 TB external Firewire drive named PI-367 is partitioned into two partitions, named MacBackup and PCBACKUP, which show on the desktop.

If you look at the bottom of the Disk Utility window, there is “Partition Map Scheme: GUID”
When a drive is partitioned, the master Map of how the drive is organized has to be written in a specific way for the computer to be able to use it as a Bootable (System) drive.  For an Intel based Macintosh, the Partition Map has to be GUID. For a Windows machine, the Partition Map must be MBR (Master Boot Record) and for an early PowerPC based Mac, it needs to be Apple Partition Map.  This is why I recommend that you Partition a new hard drive yourself when you install it, rather than let the Apple software do it automatically.  A drive that has been formatted for a Windows machine will have a Master Boot Record Map, and reformatting (initializing) the volume to HFS+ will not change it to GUID partition map.  Later on, when you decide to install OSX on the drive, perhaps because you need it to boot a Mac that is having issues, you’ll find out you can’t install on it, because it is not GUID.

To Partition a drive, start by launching Disk Utility from your Applications > Utilities folder (suggestion: I keep mine on the Dock all the time)

Select the name of the drive itself in the left hand window. (not the name(s) of the volumes on that drive, if any.  The drive name will be on the left, and any Volumes will be indented under the drive name on the list). Choose Partition from the tabs.
I am partitioning a new Buffalo external USB drive. It already has a Windows MBR partition on it already, which I Want to delete, so I am choosing the name of the drive itself (1 TB Buffalo External) in the list. MAKE ABSOLUTELY SURE YOU HAVE THE CORRECT DRIVE SELECTED BEFORE PROCEEDING!

Next, I will choose a different Volume Scheme: which will force the partition map to be remade. Clicking on Current, I can change this to 1 Partition from the drop down menu. Once I have changed the Volume Scheme, I can now click on Options, and this is the important place for me to change the Partition Map from MBR to GUID Partition Table so it can be a fully fledged Mac drive.

Click OK on the option box, optionally you can fill in a name that you want the Untitled 1 Volume (partition) to be called  (although you can do this any time later). Leave the Format under Volume Information at the default Mac OS Extended (Journaled) and hit Apply.  It will warn you that the existing volume is going to be destroyed, if it had one. Proceed with that.

I now have a new USB disk volume, “Untitled 1” on my desktop in a GUID partition and formatted Mac HFS+.