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I have an external HDD, and a pendrive connected to my Macbook Air. While monitoring the disk usage of my internal SSD, I started to copy a large file directly from one to the other, and noticed a peek in disk usage of the SSD.

As far as I know, the internal drive should not be involved in the copy process, and I know that the peek could have been caused by another process. Still, I'm interested if there could be any role of the internal drive in a case like this.

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By definition, there should be no direct involvement of the internal storage when copying data between two external storage devices, for exactly the same reasons that there should be no involvement for one external storage device when data is being copied between a different external storage device and the internal storage device. Namely, the only devices that matter are the two between which data is being copied.

However, there is at least one way it could be indirectly involved. If, for some reason, you were actively using a significant majority of the system's memory, and macOS's memory compression was unable to create enough space in RAM, you may have had to have data paged out to disk from RAM to accommodate the in-memory buffers used to copy the data from one disk to another. As the space used for paging data out to disk is almost invariably on internal storage, you would ahve thus seen a spike in disk usage when this happened.

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