Disk Utility: difference between Device and Volume?

Discussion in 'OS X El Capitan (10.11)' started by ericddd, Nov 3, 2015.

  1. ericddd macrumors newbie

    Jun 2, 2008
    Hi. I ran Disk Utility on an external USB 3 drive. First, by clicking on the device name "Toshiba Drive" it does its thing and the drive passes. However, when I click on the Volume - 3TB Logical Volume OS X Extended - First Aid fails - message is "First Aid process has failed. If possible back up the data on this volume".

    Also, an onscreen message, "You can still open or copy files on the disk, but you can't save changes. Backup and reformat as soon as you can".

    Question is why does it pass on device but not volume. What's the difference?

    I had just moved my media files from an older drive this one. For a day, it seemed fine. Should I reformat and then move all my media back to it again, or is this a bad drive?

    Not sure if this belongs here or in the Peripheral thread, if so, sorry about that.

  2. leman macrumors G3

    Oct 14, 2008
    A device is a piece of hardware, such as the physical drive itself. A volume is a logical structure that describes how data is stored. This structure can span multiple devices (in case of Fusion Drive), but is usually confined to a single one. A useful analogy is to think of a device as a storage room and a volume as an card index that tells in which part of which room to fact which data. In this, the device and the volume are principally different. It is possible that the disk is perfectly fine, but that the volume (the data index) has been corrupted beyond repair.

    Although, in your case I am a bit confused because I was always under impression that running a check on a device will also check its volumes. At any rate, I recommend that you backup the affected disk and completely reformat it.
  3. Ebenezum macrumors 6502a

    Mar 31, 2015
    Disk Utility can't repair all errors. More advanced software such as Techtool Pro or Diskwarrior are more likely able to repair the disk without having to reformat it. That is assuming problem is only in the filesystem and not in the drive itself.

    If the problem happens again after reformatting drive it's likely that the disk is defective. You might want to check the disk SMART info just to be in the safe side with software such as SMART reporter.
  4. CoastalOR, Nov 4, 2015
    Last edited: Nov 4, 2015

    CoastalOR macrumors 68020


    Jan 19, 2015
    Oregon, USA
    ericddd: +1 for Techtool Pro and Diskwarrior. I have used both and they definitely can fix problems that Disk Utility can not fix. My favorite is Disk Warrior.

    Ebenezum: I will admit that I'm not familiar with SMART reporter, but I did not think that external drives via USB would support SMART info.
  5. Ebenezum macrumors 6502a

    Mar 31, 2015
    Normally external USB drives won't support SMART info but its possible with https://github.com/kasbert/OS-X-SAT-SMART-Driver

    I have used it with Mavericks and Yosemite without problems, I have not tested if it works with El Capitan.
  6. Weaselboy Moderator


    Staff Member

    Jan 23, 2005
    Normally when you select the device up top it would check the whole volume, but you have a "logical volume" present and just selecting the device does not check that. Think of a logical volume as sort of a virtual physical disk drive.

    If you run the command below in Terminal to will see that either your drive is configured as a cores storage volume or you have FileVault turned on. Both create logical volumes like this.

    diskutil cs list
    I would remove any data to an external drive then remove the logical volume and erase the entire disk then copy the OS and data back and start over. It may be that you have the beginnings of a disk failure there.

Share This Page