| Journaling provides the most benefit to the boot drive, because the structure and layout of the boot drive as well as the files changes much more frequently than secondary hard drives. It is still beneficial to turn it on for external and secondary drives, simply because if your machine crashes in the middle of a write operation to one of those secondary drives, there's less of a chance of drive corruption, since the journal on that drive can help return the drive to a previously known good state.
In other words, if the directory of the drive is being changed, like during a write operation, and it is unexpectedly interrupted, the journal on the drive can help to correct the directory, preventing data corruption/loss.
It's just more "active" on the boot drive because of the frequency of writes to the boot drive.
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