when using Disk Utility to create a RAID 0 of two external HHD's, it asks to choose a Chunk size. the options are: 16K, 32K, 64K, 128K, or 256K.

i would like to discover what another two disc external RAID 0 drive has as it came from the factory in order to know which i should choose. but i can't find out what that is.

Disk Utility doesn't say, nor in its "Info" window.

i even tried using the Terminal command of: diskutil info /dev/disk3 , but that does not show the Chunk Size either.

are there any other Terminal commands or free Mac apps that would show this info?


fyi: using macOS 13 Ventura on a Macbook Pro M1 Max.

i have an older Western Digital My Passport Pro with a Thunderbolt 2 interface (using an Apple Thunderbolt 2 to 3 adapter/dongle). it's the one with 2 HHD's in it.

it came setup as RAID 0 but formatted in "Mac OS Extended (Journaled)". i want to reformat it to use "APFS" instead.

so using Disk Utility, i deleted the RAID and reformatted the drives. then upon using Disk Utility's RAID Assistant, i selected the RAID type to RAID 0 and selected the two newly reformatted drives. then on the next screen (Set Properties), under where you choose the Name & Format, there's a setting to select a "Chunk Size".

i've never seen this option before (granted it's been many many many years since i needed to re-RAID something), and i can't figure out which option to select...

the Chunk size options are: 16K, 32K, 64K, 128K, or 256K.

everything i've found to read on the subject says that's determined by how big the files you're working with are. to choose a small K when working with large videos & graphics files; and to choose a large K when working with smaller text based files.

but what if you have a mix for ALL kinds of files & sizes?

what would be the optimal Chunk size for a general purpose drive?

yes, i could just choose the middle one (64K) but i don't know if that's optimally average or if leaning to the left or right of middle would be better for such a mix of file types & sizes.

i did contact WD support and instead of just telling me what Chunk Size the drives came from the factory set at, their 'only' solution is for me to download/install their disk utility app. i had problems with that app working well two computers ago, so chose to not install it on my previous & current systems. i would rather a solution to be knowledge rather than 'another' app to clutter my mac innards just for this purpose. thanks.

  • This has now been reopened over on SU. It's not good to have the same question on more than one site. It would be wise to delete the other one if you think you'll do better on here [which I think you probably will, tbh].
    – Tetsujin
    May 22, 2023 at 15:45
  • Chunk size isn’t something that comes “from the factory” as it’s something defined when creating the array. Also see this related info: apple.stackexchange.com/a/406527/119271.
    – Allan
    May 22, 2023 at 15:48
  • @Allan, for external drives like the one i mentioned above, that has 2 disks in it, it came out of the box setup as a RAID 0, so when the factory set it up, they must have chosen a chunk size (as i understand it). May 22, 2023 at 20:20

1 Answer 1


The optimal RAID chunk size is not something you can "discover" from the drive itself.

The optimal RAID chunk size is a function of how you use the drive over longer periods of time.

From the information you have given here - for all practical purposes and intents - just select the middle value (64k) and be satisfied with that. You're most likely not able to feel the difference between 32k, 64k or 128k in practical use.

If you know what kind of use you're going to see very specifically - you can determine the optimal RAID chunk size. For example if you have a system where the future disk operations are well known, and you're setting up 10 of those systems - you can set up the first, do measurements, and figure out the best setting to apply to all systems afterwards.

For a small home user setup - it's not really worth the time that needs to be spent on analyzing and choosing.

For general advice on choosing small or large chunk size, see this older answer of mine:


  • 1
    @jkseogaard, THANK YOU THANK YOU THANK YOU for your thoughtful reply. that makes sense & calms my confusion. much appreciated! May 22, 2023 at 20:21

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