Question ExFat Format - Best and Smallest Allocation Unit Size for HD for Mac and Win?

Sep 15, 2021
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I'd appreciate anyone's help - or anyone's experimental results - in regards to this.

I am currently using a PC (Win 7-10) - and I'm planning to get a Mac.

1) I plan to use the PC's Command Prompt for the formatting - thus, what is the smallest allocation unit size which I can use when setting up external hard drive (HD) with an exFat format ?

2) Is a full format necesary to change the allocation unit size? Or, is a quick format okay?

In my online 'research,' a couple of writers claim that 128Kb is the lowest which will allow a MacOS to read/write the HD. However, a couple of other writers claim 64Kb or even less should work. There are not a lot of articles on this.

My backup HD is 4TB - but I want to maximize the space on the HD - so, though Win may default at 256Kb for the allocation unit size for exFAT, this creates a lot of wasted space (because tho I have big files, I also have a lot of small files).

Again, any help and advice is appreciated !
 

ex_bubblehead

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Rule of thumb is a half-a-cluster of slack/wasted space per file, which means that this is much ado about nothing. Unless you have many hundreds of thousands of files smaller than a cluster the default cluster size is more than sufficient. And, you'll find that a great many new drives use only 4K clusters, which makes your question moot.
 
Sep 15, 2021
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I recently formatted new 4TB drives (out of the box) to NTFS and exFat on my PC (Win).

Supposedly, Win's default allocation unit size (AUS) for the NTFS format is 4Kb. And supposedly, Win's AUS default for exFat is 256Kb. After I tranferred the files, I noted a LOT of wasted space. Yes, I have a thousands of small files.

Since my understanding is that it's necessary to format the drive to exFat, what should I choose for the smallest (and compatible) AUS - or, which AUS did you use when you recently formatted a HD which is compatible for both Mac and Win ?

Did you do it on the Mac - or on the PC (Win)?

Many thanks for your sharing what you experienced or an article which you may know of.
 
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ex_bubblehead

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Moderator
You'll save yourself much in the way of headaches later by simply going with the defaults. I seriously doubt that the slack space is all that large unless you have literally hundreds of thousands to millions of small files. Anything less and you're just chasing rainbows.
 
Sep 15, 2021
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The AUS default for exFat on the Mac is either 128 Kb or 64 Kb - does anyone know?

The AUS default for exFat on the PC (Win) is supposedly 256 Kb - but a lower number can be used via the Command Prompt

Since I have a PC (Win) and indeed, a lower AUS can be used... I'd like to use the optimal number.

Yes - in transferring my larger files, I could ignore the difference - but after transferring thousands of smaller files - the difference is noticeable.

(1) Comparison of NTFS versus exFAT drives: Size versus Size on disk

(2) Properties of the drives (from NTFS drive to exFat drive) in terms of used and free space - so, there's almost a difference of 60 GB and I haven't finished transferring all the files yet.

Thus, if any wonderful member knows or has had experience with this, I'd appreciate hearing from ya !
 

ex_bubblehead

Champion
Moderator
The "Optimal" number is the one that Microsoft has chosen. You will not gain a thing by changing this. In fact, if the cluster size on disk does not match the page size (4KB) you will take a massive performance hit. And how, exactly, did you determine this 60GB number for wasted space? That's a most improbable number, unless you have several hundred thousand to a million plus files of only a few hundred KB. Not likely.
 

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