Drive Capacity is reported by percentage. Finder uses 1000 to equal kilo instead of 1024 (210) which on larger drives may not match our reporting. If iCloud Drive is enabled, the Monitoring Client may report the drive at capacity, while Finder shows free space. This article addresses both issues.
Store in iCloud/iCloud Drive
If the Store in iCloud option in is enabled on the computer, Apple will report potential free space (Apple menu > About This Mac, then click Storage) versus the actual free space on the drive at the command line (df). When storage space is needed, only the files you recently opened are kept on the computer. Apple flags files that could be deleted to make room because the file(s) could be resynced from iCloud.
With this scenario, Apple tends to keep the drive capacity at 95%. We feel that 95% is too much occupied space, because it does not allow sufficient space (usually 10% of total drive capacity) for cache and virtual memory for the computer.
Learn more about Documents in iCloud Drive https://support.apple.com/en-us/HT206985 and Managing Storage on your Mac https://support.apple.com/en-us/HT206996.
Volume Size
In Mac OS X 10.6 and above, the Finder uses the kilo = 1000 standard when calculating file and volume sizes. However, different tools report different file sizes and volume usage, such as the Info details and Quick Look.
Watchman Monitoring uses the terminal command df when checking the root drive capacity. The kilo = 1024 standard is used by df.
Regardless of the numerical size reported, the warning is triggered based upon the percent of drive space used, which is calculated correctly.
Learn more about how macOS calculates size information: http://support.apple.com/kb/TS2419
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