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I have a folder in Samsung Portable SSD T5 model.

When I connect the SSD to MAC and right click the folder and select Get Info to find the folder size then I see the following: 170 GB (410 GB on disk)

What do those 2 values mean? --Size and Size on Disk

Why is there such a huge difference (I know that my folder only has content of size 170 GB so why is it occupying 410 GB on the disk)? --Because it depends on the Allocation unit size and FileSystem in which the SSD is initially formatted to use.

Is the 240 GB (410 - 170 = 240) unusable right now unless I format the SSD? What is the suggested FileSystem and Allocation Unit Size that I can format the SSD to use, so that it is compatible with both Windows and MAC OS (Windows 10 and MAC Catalina)

firstpostcommenter
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  • What filesystem are you using? – Attie Jun 30 '20 at 13:28
  • I used the default one provided by the SSD. I think it is `exFAT` as it was compatible with both MAC and Windows OS – firstpostcommenter Jun 30 '20 at 13:38
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    @firstpostcommenter It's due to the Allocation Unit Size selected when drive was formatted... drives should never be formatted with a size greater than `4KB` _(there are exceptions for specific use scenarios, but in general, `4KB` should be the max set )_. I personally would never recommend exFAT _(default allocation is `128KB`)_, as Windows has ext2/3/4 support via Paragon's [LinuxFS](https://www.paragon-software.com/home/linuxfs-windows/). If exFAT is a must, use lowest available Allocation Unit _(`16KB` on Windows - `1KB` file takes up `16KB` of disk space)_, again, not recommended. – JW0914 Jun 30 '20 at 13:47
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    Does this answer your question? [What is the difference between “Size” and “Size on disk?”](https://superuser.com/questions/66825/what-is-the-difference-between-size-and-size-on-disk) – Kamil Maciorowski Jun 30 '20 at 13:56
  • @JW0914: "drives should never be formatted with a size greater than 4KB"--Oh ok. I have never formatted or selected any specific size and I dont even remember selecting an Allocation unit size during the initial setup of SSD. I just went with the default options prompted by Samsung. Now, is my 240 GB (410 - 170 = 240) lost unless I format my harddisk and change the Allocation unit size? Secondly, which file system should I use for SSD to support both Windows and MAC OS if not for exFat? – firstpostcommenter Jun 30 '20 at 14:41
  • @KamilMaciorowski: Now I understand the difference between "Size" and "Size on Disk" but I am still looking for how to solve the issue because there is too much space difference between "Size" and "Size on disk" right now. I want to know the filesystem and allocation unit size that need to be used and how to do that? – firstpostcommenter Jun 30 '20 at 14:46
  • @JW0914: Does number of files in a folder influence this huge difference between the Size and SizeOnDisk? The folder I have has too many files and subfolders inside it. Incase there is some other folder of same size but has less number of files and subfolders inside it would it have not created such huge SizeOnDisk or is it the same regardless? – firstpostcommenter Jun 30 '20 at 14:58
  • @firstpostcommenter FAT-based filesystems have always been inefficient, and I always recommend avoiding them unless it's necessary. Directory tree size is not what's causing the disparity, the Allocation Unit Size is... at a default size of `128KB`, that means that any file not divisible by `128KB`will take up an additional `128KB` file block beyond what the file size is, so a `257KB` file would actually take up `384KB` of storage on the disk. To resolve, backup the dri've contents to another drive, reformat to ext4, and move everything back (use LinuxFS - 1st comment - on Windows for ext4). – JW0914 Jun 30 '20 at 15:46
  • @JW0914: Is ext4 supported in MAC by default without requiring any softwares? How to find out the filesystem of my SSD, I want to confirm if it is indeed exFat or something else? – firstpostcommenter Jun 30 '20 at 16:19
  • Disk Utility https://cdn.osxdaily.com/wp-content/uploads/2023/01/determine-file-system-drive-mac-disk-utility-1-610x373.jpg, but now this is turning into a different question. – Joep van Steen Jun 07 '23 at 14:40
  • The gist is: probably exFAT with large cluster size. Solution: backup what's on the SSD and reformat with smaller cluster size. Which file system to pick is a whole new discussion and material for a separate question. – Joep van Steen Jun 07 '23 at 14:41

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For unknown reasons, Sandisk formatted their Extreme SSD with a 1024kb allocation unit size, which means that every file you put on the disk uses at least one megabyte of drive space.

Windows would default to 128kb for a disk of this size, and that's what the Samsung uses for their 4tb T7 Shield drive, for example.

EricLaw
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