Erased 32GB USB flash storage with Disk Utility's "Secure Erase" feature, one pass of zeros. Disk Utility now reports that there is only 26GB of my drive available. Not sure what is going on here, but I'd like to know where my 6GB of space went.

Update: Zeroed the drive using DD, created new partition using Disk Utility, resized the partition and created a new one. It appears that for each of my partitions created, 6GB of the available space is taken up by something. The hidden files (Trashes, Spotlight, and fseventsd) do not account for this usage. This is really baffling and annoying, and any clue as to what is happening would be very appreciated.

Edit: Pictures to clarify:

Problem occurs on more than one drive, I think it has something to do with either journaling or local time machine backups.

Info when the partition is selected in left column

Info when drive selected in left column, partition tab


3 Answers 3


This is likely coming from one or two things (or both).

Partitioning a disk, any disk, takes up drive space. In the megabyte range it is not even noticeable. In the gigabyte range it can be a substantial byte (sorry...) out of the drive.

Also computer makers and Drive/RAM makers often calculate bytes differently. O/S companies (Apple, Microsoft, etc...) call 1K 1024 bytes. Drive/RAM makers call 1K 1000 bytes. If you do the math it looks like you "lost" a lot of space when it is just different conventions.

Also formatting a drive will map out bad blocks. So depending on how many bad blocks there are that can reduce the size of the formatted volume as well.

  • 1
    I don't think that the partition map nor bad blocks accounts for the 6GB, as it's taken away from every partition I make on the drive.
    – TGYK
    Jun 15, 2015 at 21:32
  • 2
    Mac, specifically, uses base 10 to display drive capacities, so the number shown in OS X should be very close to the manufacturer's stated capacity. Unlike Windows, there is nothing "lost" in conversion between base 2 and base 10. Even if there was a conversion, 6 GB is far more than the expected difference.
    – tubedogg
    Jun 15, 2015 at 21:33

How old is the drive?

Flash memory has limited write cycles, and it will slowly change into a read-only device. If you have been writing stuff to the drive for a few years it is quite possible some blocks have reached their write limit and the controller has marked them as unusable. Cheaper drive -> fewer cycles.

Also note that the one-pass-of-zeros secure erase method doesn't really work on solid state media, and it's another hit on the drive's write cycles.

  • The drive is not too old, fairly unused. I think that the issue is something to do with journaling. When disabled, I'm able to make partitions smaller than 6 gigs but cannot resize them. Disk utility is great, but kinda confusing sometimes.
    – TGYK
    Jun 16, 2015 at 1:10

What does Finder report? How about turning Time Machine off and then back on? (deletes local snapshots)
Also maybe try Disk Utility>First Aid>Repair Disk
Have you tried copying a large amount of data to these partitions? Copy something big to get to 90% maybe and then copy more and see when it says it's full. Sometimes Disk Utility reports incorrect capacity stats from what I've read.

  • I am able to copy files to this portion of "Occupied" space no problem, but I cannot partition the space any smaller, even after the Time Machine toggle.
    – TGYK
    Jul 2, 2015 at 2:28

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