Allocation Unit Size
When files are written to a Disk, the Allocation Unit Size specifies in what size increments that data is recorded. For example if a volume’s Allocation Unit Size is set to 1 MB, the smallest amount of data that can be written to that volume is 1 MB. Even if a file is less than 1 MB, that file will still take up 1 MB because of the Allocation Unit Size.
NAS OS 4.1 and earlier use an Allocation Unit Size of 1 MB.
NAS OS 4.2 and later use an Allocation Unit Size of 4K.
This means that when copying small files to a Seagate NAS running NAS OS 4.2, they will take up less space on the volume compared to devices running NAS OS 4.1 and earlier.
Important Note: Any files stored on the NAS during the update to 4.2 will still take up the same amount of space based on the previous Allocation Unit Size. Any files added after the update will use the new Allocation Unit Size. The volumes on your Seagate NAS devices do not need to be formatted to use the new Allocation Unit Size, this will happen automatically on existing volumes after the update to 4.2.
You can convert the old files by moving them to a different location, either on the network or to an external USB DAS for example, and then moving them back to a Share on the NAS. It is strongly advised to have at least 2 copies of the data available during this process, in case of accidental data loss.
Situation - You’re trying to copy a large amount of small files to a NAS OS 4.1 or earlier Share. On the computer these files might be reported as 100 GB total for example, yet once they’re copied to a Share they’re taking up considerably more space. In some cases this leads to not enough disk space errors even though there seems to be plenty of space on the NAS volume for the files.
Reason - NAS OS 4.1 and earlier devices have a 1MB Allocation Unit Size vs 512, 1K or even 4K which are available options when formatting a regular external or internal HDD. Because of this, when a small file under 1 MB is copied to a NAS OS 4.1 or earlier Share, it will still take up 1 MB of space on the volume.
When copying 1 file at a time, this effect is hardly noticeable, however when a large amount of small files are copied the difference can be apparent.
Binary vs. Decimal
Windows calculates capacity based on a binary system, while the NAS OS calculates the same capacity based on a decimal system. This is the same reason Windows only shows 931.39 GB capacity for a 1 TB internal or external drive, for example.
Simply put, decimal and binary translate to the same amount of storage capacity.
Let's say you wanted to measure the distance from point A to point B. The distance is 1 kilometer or .621 miles. It is the same distance, however it’s reported differently due to the measurement.
There was a similar difference in older versions of OSX, however this has not been the case since OSX 10.6.
It’s the same principle for internal and external drives connected to a Windows computer, see this article for more details: “Why does my hard drive report less capacity than indicated on the drive's label?”