Mac-formatted external hard drives are often priced higher than their Windows counterparts and are not as widely available. You can repurpose any hard drive to work with your Mac. Macs running OS 10.5 and higher -- Leopard to Yosemite -- include the Disk Utility program that allows users to check and repair disks and drives and to format or erase drives.
Connecting External Hard Drives
External hard drives can be any size, ranging from portable USB thumb drives to large drives connected using USB, FireWire or Thunderbolt cables. Older Macs provide USB and FireWire connections, and newer Macs include USB and Thunderbolt ports. FireWire, USB 3.0 and Thunderbolt connections provide faster data transfers than USB 1 or 2. If you're unsure of which model you have, choose an external drive with a USB 3.0 cable, as this type of connection is compatible with older USB ports.
Use Disk Utility to Format an External Drive
After you connect an external drive to your Mac, open the Applications folder from the Finder window. The Disk Utility application is located in the Utilities sub-folder. Launch Disk Utility and then click to select the external hard drive listed on the left. Click the "Erase" tab, choose a volume format from the drop-down menu and then type a name for the drive. The Mac OS X Extended volume format is optimal for Macs; the Journaled option enables the system to log and keep track of files. The MS-DOS FAT32 or ExFAT volume formats are compatible with Windows computers.
Securely Repurpose a Hard Drive
Use the Security Options to format a previously used hard drive. In Security Options, move the slider to select how many times you want the system to erase over the data. During the formatting process, the system erases and writes over old data, preventing old information from being accessed through data recovery programs. So, while it takes longer to erase files, the security of the hard drive is increased. The Zero Out Data Option is the fastest formatting option and provides the most basic security as it erases unused disk space. The 7-Pass Erase or 35-Pass Erase require more time to format a drive to zero out all data.
Option: Partition the Hard Drive
Create sections called partitions in a large external drive. Each partition appears on your computer as a separate disk. Each partition can be managed as a separate hard drive for organizing folders, storing backup files, or as a hybrid system containing Mac- and Windows-formatted drives. In the Disk Utility, select the external drive from the list on the left and then click the "Partition" tab. Select the number of sections or partitions you want to set up on the hard drive and type a name for each partition. Choose the Volume Format for that partition. Click the "Apply" button to set up the partitioned hard drive.