I want to know if using Mac OS Extended Journaled is the right choice for a MacBook Pro that would only be connected with the external hard drive at night. It does not make sense to me to have it journaling every action of a Mac that is not always connected.

I also want to use the same external hard drive for Time Machine backups. What is the right way set up an external hard drive to do file backups with a MacBook.

I am confused on how a journaled file system and Time Machine aren't related processes. Do I do both or do they both happen together?
How do they work?

3 Answers 3


I will try to explain journaling as I see it. I probably will not give the process completely right, but you will have a better idea of how journaling works.

Let us say I want to write some new data to the journal file system. The steps would be something like the following.

  1. The operating system writes a new entry to the journal. This entry states the I want to write the new data. The operating system also includes what every information would be needed to undo the operation.
  2. The operating system marks in the journal that it finished making the journal entry.
  3. The operating system now goes through the process of writing the new data to the drive.
  4. The operating system marks in the journal that it has completed the task stated in the journal entry. The space used in the journal by this entry can now be reused.

If the system crashes during these steps, then the following happens during restart of he computer.

If the computer never completed step 2, then the journal entry can me marked completed so the space can be reused. In other words, the task of writing the data was never started, so the journal entry can be ignored.

If the computer completed step 2, but not step 4, then the information stored in the journal entry is used to return the file system to the state before the journal entry was made. After this is completed, the journal entry can be marked as completed so the space can be reused.

So journaling is a temporary operation that helps prevent the corruption of a file system. This slows down access to the drive, usually this cost is outweighed by the benefit of the protection.

Time machine backups are a permanent backup of changes made to your drive. You can use the backup to restore parts or all of your file system to a previous state.

  • Journaling protects the file system. It will delete a file if things crash in the middle of an operation to let the system be "consistent"

  • Backups protect the files. You can go back and get the file when the journaling or a mistake or a crash renders a specific file broken or gone.

See What are the differences between journaling HFS+ and non-journaling HFS+? for what chances between a non-journaled filesystem and a journaled one. It's a bit technical, but that's where you are when you get down to file system journaling.


A journaling file system monitors low-level file system operations like creating, deleting or moving files and folders on a volume. By logging all atomic tasks in the journal and deleting them after the operation has been finalized successfully, it always tracks the current state of the file system. Every volume with a journaling file system contains its own journal.

Since all the operations need several steps to finalize, a power loss or a system crash may leave the file system in a non-consistent or unknown state. After a crash or a power loss the presumably corrupted file system has to be checked and repaired if necessary. The journal may dramatically decrease the time needed to complete the task by providing informations about open operation.

The journal usually doesn't contain any data of a particular file but only metadata.

Backing up a volume or parts of it with Time Machine creates a duplicate of the chosen files and folders on a separate volume. If you lose your Mac, delete a file or folder accidentally or the main hard drive dies you will still have a copy of the data on your backup volume and may recover it.

A power loss or crash can affect all mounted volumes, including the main volume and the backup volume. A crash while backing up files on a backup volume (which is nothing more than file system operations on a second volume) has the same consequences as on the main volume: the file system can be non-consistent. After a crash both volumes are checked for errors.

To keep track of all file system operations (even on the backup volume) and decrease potential repair times, enabling Journaling on a backup volume is recommended.

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