How to choose a format for an external drive in OS X

USBDriveIconXFormatting an external drive to work with your Mac can be done in several ways. While for the most part drives ship in formats that are universally recognized among operating systems so data can at least be read, there are times when you may wish to wipe a drive and set it up to be most compatible with your Mac. On the other hand, you may have a drive you have primarily used with your Mac, but now wish to use it with a Windows system. In these cases, it helps to know how best to format your drive.

There are two major ways to format a drive in OS X, and both use the technologies behind Apple’s Disk Utility. The first is the Disk Utility tool itself, which is the easiest and most straight-forward tool, and the other is the “diskutil” command in the OS X Terminal, which may be more suitable for power users. In both cases, you can target a drive to partition it and format it according to your needs.

For most intents and purposes, there are two parts to keep in mind when formatting a hard drive: the format and the partitioning scheme. The format itself is the layout and structure of the drive’s index and database, and is similar to a table of contents in a book. Once data is written to disk, most of the heavy lifting is done by these index files. It is they which allow for permissions restrictions, and file naming, among other details.

In OS X, you have three main formats which you can access: Variations of Apple’s Hierarchical File System (HFS) often named “Mac OS X Extended”, the File Allocation Table format which is listed as MS-DOS, and the extended File Allocation Table (Ex-FAT) format. OS X also supports Microsoft’s New Technology File System (NTFS) format; however, by default OS X can only read NTFS.

OS X Extended

This is OS X’s native drive format, and should be used whenever you have a drive that you intend to use only with Mac OS systems, including backup drives, scratch drives, or extended storage you are using with your Mac. It is best if you use the drive for any of the following:

  1. Mac-only use.
  2. Full-disk encryption.
  3. OS X Boot and utility drives.

Case-sensitivity
 With OS X Extended formats, you have the option for case-sensitivity. By default OS X is case-insensitive meaning it only cares about the alpha-numeric sequence of file names, and does not distinguish between case. With case-sensitivity, the OS will see a file called “MyFile.txt” as being different from “myfile.txt” so they may exist in the same folder. This feature is only useful for special software considerations, and for ease of use by not being confused between files based on nuances of character capitalization, its best to leave your drive in a case-insensitive format.

Journaling
 Note that for these two you may see “Journaled” as an option. This is an extension feature to the format that ensures files are fully written to the drive in a location called the Journal before they are committed as stored data, which helps guarantee file integrity if there is an interruption, such as a power outage. In such cases, the half-written journal contents is discarded and the drive’s original data is restored. This feature lends to more robust data management when this format is used in OS X.

MS-DOS (FAT)

This is the default format for many external hard drives, as it’s both readable and writeable by many operating systems, including OS X and Windows. It has limitations, such as not being able to write files larger than 4GB, and no support for file permissions, and no journaling support, but in most cases its cross-compatibility is appealing for temporary storage. This will be your most universally compatible format, best for the following:

  1. Most common file sizes.
  2. Temporary storage.
  3. Multiple operating systems.

NTFS

This format is supported as read-only in OS X, so you may read from a drive used on a Windows machine. It is as robust of an option for Windows as Mac OS X Extended is for OS X, supporting encryption and permissions features, and other nuances that give extended control to systems running Windows; however, licensing and politics has kept Apple from implementing full support of this format, so you will not be able to choose from it as an option.

ExFAT

If you are at all finding limitations with the use of FAT formats for moving files between Windows and OS X, mainly because the files are too large (e.g., movies and other streaming data) and are giving you an error, then you can use the ExFAT format. This is an extension to the FAT format that allows for larger file sizes to be used, access control permissions restrictions, among some additional nuance metadata handling features. Overall, the use of ExFAT is best for the following:

  1. Later versions of Windows (beyond Vista), and OS X.
  2. Larger file sizes.
  3. Desire for file access control restrictions.

15 thoughts on “How to choose a format for an external drive in OS X”

  1. B. Jefferson Le Blanc

    I found ExFat used on a NAS that I had to set up for a client. The built-in software would not recognize FAT 32 or HFS formatted drives. It was exceedingly tedious to use. I don’t know how common this is on NAS systems, but it was very off-putting at the time and I’ve avoided them since.

  2. wally

    ExFat is also not journaled. If drives aren’t explicitly ejected, there will be errors evident next time they’re plugged in. Sometimes OSX cannot repair them.

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