When purchasing a new hard disk for your Mac, even if it is SATA/ATA(and this means internal) or FireWire (external), the box the hard drive is in will tell you if the drive works both on Macs and PCs. New hard drives might have a 2 Gb file size limit, but there are hard disks which have no limitations. Even if you intend to partition or not the hard disk you should format it before using it. If you got a new FireWire disk and you want to use it both on Macs and PCs, then it doesn't need any format.
Disk Utility is the application which helps you perform disk-related tasks in Mac operating system. The application has been developed by Apple, and the possible tasks are: creation, compression conversion or encryption of disk images, on a large range of formats, mount, unmount and eject disks, verifying disks integrity and sometimes repairing them, disk erasing, formatting and partitioning, modifying RAID sets, restoring volumes from images, burning disks in HFS+ format and many other utilities. This utility can be accessed from Mac OS X command line too, using the diskutil and hdutil commands. This utility has been updated along with the OS development, and in Mac OS X v10.5 Leopard it has the great ability of creating, resizing and also deleting disk partitions without erasing them, and this process is known as live partitioning.
For formatting a volume you should go to "Erase" section in the Disk Utility window, and then choose the volume you want to be formatted, then the format you would like on it, and edit the "Name" field. The Security Options also have to be modified, and you can choose between: "Don't Erase Data", "Zero Out Data", "7-Pass Erase", or "35-Pass Erase". All these options are explained, and you should pick one according to your needs. If you followed all these steps, the process is ready to start and all you have to do is click on "Erase". The status bar will appear and it shows the progress of the formatting process. When the format is complete, you will notice the disk remount in Disk Utility's window.
The formatting/partitioning process is the same for Tiger and Leopard systems, but the Disk Utility application has different interfaces for those two types of systems. In Leopard, there are no default drives selected in the left column. The tab "Partitioning" is not available as it is in Tiger. You should select the hard drive you want to partition/format in the left column. The partition tab will appear only after selecting a hard drive, and there you have the options to edit the Name and the Format sections, and you should tick a box , "Mac OS 9 Drivers Installed", if it really is so.
In the partition tab you should select the number of the partitions you want to edit, and then you can customize the settings, and you can also choose which partition will be Journaled (this means that it will serve as boot volume. If you want to store data or media, you should also choose Mac OS Extended and you can change the size of the partitions by dragging the button found in the Volume Scheme section. After you set up all the things required, click on "Apply" and the process starts. In Tiger, the process is very similar, just there are some differences in selecting and naming the volumes and in the journaling settings.