IMac – Difference in disk usage MS DOS vs Extended Journaled

disk-formatdisk-spaceexternal-diskhard driveimac

Searches on Google and the Apple forums didn't get me any answers or couldn't clarify this for me, hence this question.

I have two following external USB hard drives connected to my iMac:

  • "Lacie1" with a capacity of 499.64GB, formatted as MS-DOS (FAT32)
  • "Lacie2" with a capacity of 499.76GB, formatted as Mac OS Extended (Journaled)

I copied all of the contents from Lacie1 to Lacie2 and to see if everything was copied well, I opened the show info dialog of both drives and noticed that there is an enormous difference in used disk space between the two:

  • Lacie1: 221.357.604.864 bytes (221,36GB)
  • Lacie2: 187.714.969.600 bytes (187,71GB)

To be sure I checked how many files are on each drive and they both contain 17297 parts.

Is this because of the formatting of the drives that there is such a difference in disk usage or is there indeed a 'problem'?

Any explanation is appreciated! 🙂

Best Answer

FAT32 on a large drive has a larger block size that HFS+. This means that the small amount of "wasted" space per file is larger and therefore the same files will take up more space. It's because FAT32 only uses 28 bits for it's cluster index while HFS+ can use it's entire 32 bits.

One of the marvelous things about Steve Wozniak is that he designed a disk system for the Apple ][ that was both efficient, extensible and easily implemented and the original Macintosh File System (MFS) was built on top of his work after Wendell Sanders created a single chip implementation of Woz's drive controller the IWM chip. It was replaced by the Hierarchical File System (HFS) and then HFS+, otherwise known as Macintosh Extended.

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