Why has my external hard drive become read-only on my Mac?
Question by Kim /
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I have an external hard drive set up for backing up with Time Machine on my Mac. It has been working well for about a year. Now when time machine tries to back up it tells me the external disk is read only. The hard drive does mount to the computer and will let me open the Time Machine folder. When I am in disk utility it also sees the external hard drive but will not let me click on repair disk permissions. The only thing that I can add is that we had a power outage in the middle of the night recently and maybe that had something to do with this? Any thoughts on how to get this drive back to read and write? The drive has not been ever used on a windows machine or use on any other computer. One day it was read/write next day read only. Thanks!

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Answers (17)
  • The Social Pet

    I have an external Western Digital 500 gb hard drive, and it is hooked up to a Mac laptop running OS X.

    About one week ago, it suddenly went "read only." I can't save anything to it at all. Even documents that I have opened and closed a hundred times, now suddenly I can't close with any changes.

    It is not full. It still has 121 gb free.

    Any ideas?


  • Ike Nelson

    It's a permissions problem and giving everyone permission on that drive will solve your problem. I am awaiting the directions on just how to get there, I will pass it on.


  • Nick

    Trying to repaire my brand new external hard drive's permission. (Aplic/Util/HD/). Commands like 'repair disk permission", "verify disk", and "repair disk" are grayed-out. Te rest ( "Fist Aid, Erase, RAID, and Restore") are functional but usless (OSX Snow Leopard).

  • Tim

    Kim, I have the exact same issue. If memory serves, the issue started about the same time frame you mentioned (I can't be certain of this, but is seems about right). I wonder if there has been some kind of update? Have you found any solutions? While the like below is helpful, unlike the author I DO have a bunch of data that I don't want to loose if possible.

  • Anonymous

    My new external Mac drive is read-only. How can I fix it?

    • coolhive

      Not helpful if there is date on the drive you want to recover! Any help?

    • coolhive

      This doesn't help if you still have data on the drive that you can't access or copy off the drive. I've checked the permissions and it only says "Read Only", even tho I've used this drive on another iMac. any help?

    • ha14

      on terminal (Volumename is your volume!)

      sudo chflags 0 /Volumes/Volumename
      sudo chown 0:80 /Volumes/Volumename
      sudo chmod 775 /Volumes/Volumename
      sudo chmod -N /Volumes/Volumename

    • Coolhive

      Thanks, but I haven't learned Terminal. And unfortunately, I have a strict "Do not mess with Terminal with advice on the Internet" policy. Based on my ineptitude, not on anything you wrote or others.

      Do you or anyone have another suggestion that doesn't require tinkering under the hood?

    • Mike

      Select your external drive and press cmd + i (or rightclick > Information).

      In the new windows there should be a lock icon in the lower right, if it's closed click on it (you will be asked for your credentials).

      Once the lock is open set the permissions to:
      : Read & Write
      staff: Read & Write
      everyone: No Access
      (or Read & Write, whatever you are more comfortable with)

      Then click the 'gear' icon below and click "Apply to enclosed items…"

      However, this will only work if the drive is actually using a compatible file system.

      If the drive was previously used with Windows chances are it is formatted as NTFS which is read only by default. In that case you'll have to install a NTFS file system driver on your Mac.

    • The Social Pet

      I don't get the lock icon in the lower right like Mike is saying, but it does say, "You can only read." in the lower left.

    • Ike Nelson

      Change the permissions to read and write

  • Mike

    As far as "Repair Permission" goes this option is only available to the drive/partition where Mac OS X is installed. It relies on the "Receipt" files installers place on the computer and repairs the permission according to them.

    One thing I do suggest to run "Repair Disk" on the external drive to make sure it's file system structure is in tact.

    As for the permissions you will have to set them manually. The only other option is to delete the entire Time Machine storage and start over.

    Time Machine stores the backups in the "Backups.backupdb" folder. Select it and press cmd + i (or rightclick > Information) and make sure the following permissions are set:
    system - read & write
    admin - read & write
    everyone - read

    Honestly I rarely do permissions via the GUI...
    Usually I do all this stuff via Terminal (/Applications/Utilities). Via Terminal the commands should be along these lines - just replace "TIME_MACHINE" with the name of the external drive:

    sudo chflags -R nouchg "/Volumes/TIME_MACHINE/Backups.backupdb"
    sudo chown -R system:admin "/Volumes/TIME_MACHINE/Backups.backupdb"
    sudo chmod -R 775 "/Volumes/TIME_MACHINE/Backups.backupdb"

    chflags nouchg ... removes a flag that locks files and folders
    chown system:admin ... sets the owner to "system" and group "admin"
    chmod 775 ... sets owner (system) read & write, group (admin) read & write, everyone read

    • Ike Nelson

      It can be done in GUI and will be much more approachable for a"average" people.

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