
Mac computers use HFS as their native file system. At 24 Hour Data we have many clients that think their external hard drive has failed, because their Mac will not mount the drive. Most External Hard Drive manufacturers like Western Digital, Seagate, Hitachi, and more Pre-Format drives with Windows file systems. By default most externals are formatted FAT or NTFS.
Macs will see HFS, FAT, and NTFS. However, if you have a Mac you will not be able to write to NTFS formatted drives without a third party application. Many Mac users get a drive from Frys, Best Buy, Comp USA, and so on that is pre-formatted FAT. When they start using the drive they have no issues. This because Macs can read and write to the FAT file system just fine.