Understanding File-System Formats

Mac OS X supports several file-system formats, whose characteristics are outlined in the following table.

	HFS Plus	UFS	MS-DOS
Compatible OS	Mac OS	UNIX	Windows
Case treatment	Case preserving by default Case sensitive available	Sensitive	Preserving
Forked file support	Yes	No	No
Journaling	Yes by default May be disabled	No	No
OK as Mac OS X Server boot volume	Yes	No	No
Visible to Mac OS 9	Yes, if not case sensitive or journaled	No	No

• Hierarchical File System Plus (HFS Plus): This is the file-system type used on Mac OS 9 and earlier systems. It is also known as Mac OS Extended File System. In Mac OS X version 10.2, you could enable journaling on new or existing HFS Plus volumes using Disk Util. As of Mac OS X v10.3 and going forward, newly created HFS Plus file systems have journaling enabled by default.

  Occasionally, HFS Plus causes problems with UNIX programs and shell scripts that assume case sensitivity. You can format a new HFS Plus volume as case-sensitive by using Disk Utility or diskutil from Terminal. Many third-party Mac applications do not work correctly with case-sensitive file systems, however, so you generally don't want to use a case-sensitive file system, especially on a Mac OS X boot volume.

• UNIX File System (UFS): This is similar to other UNIX file systems in that it is case-sensitive. The problem with UFS on Mac OS X is that it does not support forked files. For that reason, UFS is not recommended as the root volume on Mac OS X. Another feature of UFS is that Mac OS 9 cannot see UFS file systems. This can be an advantage because Mac OS 9 is less secure.

• MS-DOS: Mac OS X v10.3 and up also support the Windows disk format, MS-DOS (FAT). This feature allows you to use an external drive or flash memory device that was formatted for a Windows computer on your Mac OS X computer. Another Windows disk format, NTFS, is supported in read-only mode. Like UFS, Windows file systems do not support forked files.

• Other formats: Mac OS X supports the Andrew File System (AFS), a common networked file system that is particularly popular among educational institutions, with the help of a package available from openafs.org. It also supports the standard ISO 9660 format for CDs and DVDs.