MacOS – New securely erased drive not empty according to Disk Utility

disk-utilitymacossecure-erase

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

Best Answer

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.

Related Solutions

MacOS – Disk Utility will not let me create new partition

It looks like you partition the whole drive to be the Mac drive.

To undo that and create multiple partitions follow following procedure.

How to Partition A Fusion Drive _{Last updated January 10, 2013}

See Controlled Fusion: Partitioning for Targeted Performance.

Shown below are two partitions of the Fusion drive in Disk Utility.

To partition a Fusion drive:

Backup your stuff (two backups strongly recommended).
Boot off any working drive (e.g. one of your backup clones).
Erase the Fusion drive (because your stuff might not fit on a downsize partition).
In the Partition tab, click the + button.
Drag the slider between the partitions to size as desired.
Set a name for the 2nd partition.
Apply.
Clone your system back to the new Boot volume (if too large, move some items first to the new 2nd volume).
Copy your other data (data not going onto the Boot volume) onto the 2nd volume.
Set the startup volume to the new Boot volume, reboot.

Resizing and moving OSX partitions in Disk Utility

I recommend downloading a copy of GParted from here: http://gparted.org/download.php

Then either:

Burn that iso file to a CD and then boot from the CD.
Or, create a new partition on your disk, use Disk Utility to Restore from the iso file to the new partition, set that partition as the startup disk, and finally boot into GParted.

GParted is the king of partitioning tools.

Best Answer

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MacOS – Disk Utility will not let me create new partition

Resizing and moving OSX partitions in Disk Utility

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