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iPartition reports different sizes to Disk Utility/Finder

You may notice that iPartition reports different sizes to those shown in Disk Utility and Finder. There are two reasons for this.

First, if you are looking at free space in iPartition vs free space in Finder, the problem is that you are comparing two entirely different things; iPartition’s “Free Space” is the part of the disk that is not yet occupied by partitions. Finder’s free space is that part of a filesystem (within a partition) that is currently unused.

Second, iPartition and Disk Utility/Finder may well be using different units. Prior to Mac OS X 10.6, OS X, like most software, used “traditional” units, so 1KB is 1,024 bytes, 1MB is 1,024KB and so on. However, hard disk manufacturers have tended to use SI units, so 1KB is 1,000 bytes, 1MB is 1,000KB.

It’s pretty plain that the drive manufacturers were doing this for marketing reasons, as opposed to a burning desire to stick strictly to SI units. How can we tell? Because hard disks are divided into blocks, and those blocks have power-of-two sizes (typically 512, 4,096 or 8,192 bytes), which means that using traditional units would result in a round number where using SI units won’t. That is, it always made sense to use traditional units for disk storage. But the number in SI units is bigger, so better for marketing purposes.

How much bigger, you ask? Well, for a 20MB disk, we’re talking 948KB (slightly less than a MB, or around 5%). But for a 20GB disk, we lose about 1.3GB (nearly 7%), while for a 1TB disk we lose around 92GB (9%).

By default, iPartition uses traditional units, with their traditional suffixes. You can, however, ask it to use SI units (iPartition refers to these as base-10 units), and you can also ask it to use the official (IEC) unit names rather than the traditional names, by changing these preferences:

Unit display options in iPartition's General Preferences


Posted by alastair at 2015-Jun-26 11:06:47 UTC. Last updated 2015-Jun-26 11:06:35 UTC.