Why have I lost write permission for the external hard drive I use with Time Machine?

Chuck H July 18, 2013
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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. 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. Under the drive info’s sharing & permissions window it said ” You can only read.” , After unlocking everything is grayed out, but the permission is set to read & write. Any thoughts on how to get this drive back to read and write? Its a 2TB drive with 1.79TB on disk. The drive has not been ever used on a windows machine or use on any other computer and is use only for Time Machine backups. One day it was read/write next day read only. Thanks!

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  1. Justin Pot
    July 19, 2013 at 4:38 pm

    I'm going to echo what others have said here and recommend you attempt Repair Permissions in Drive Utility. Not sure what caused your problem, but this will likely fix it.

  2. Susendeep D
    July 19, 2013 at 10:48 am
    • Justin Pot
      July 19, 2013 at 4:37 pm

      I wrote that article, and I can tell you it's not going to help. The drive in question has never been used with a Windows computer.

  3. Jan F
    July 19, 2013 at 9:31 am

    Repair permission within Disk Utility only works for your boot drive (where OS X is installed). That is intended behavior.
    http://support.apple.com/kb/ht1452

    See if it is a permission issue:
    1. right-click "Get Info" on the drive
    2. click the lock symbol at the bottom right of the info window (you will be prompted for your password)
    3. check "Ignore ownership on this volume"

    the second option would be to fully unlock the drive

    1. open up a terminal Window

    type "sudo chflags nouchg /Volumes/external_drive"
    type "sudo chown shortname:staff /Volumes/external_drive"
    type "sudo chmod 777 /Volumes/external_drive"

    replace shortname with the name used for your homefolder, replace "external_drive" with the name of your external drive

  4. Hovsep A
    July 19, 2013 at 9:01 am

    Applications --> Utilities --> Disk Utility. select the drive you want to fix:
    click on the "X" drive, then on the button Repair Disk Permissions

  5. Bruce E
    July 19, 2013 at 4:12 am

    It doesn't sound like a permissions problem, but a mounting problem; the drive itself is probably being mounted as read-only. Check /etc/fstab to see if this is the case. You may need to use vifs to edit your fstab file to make it mount the drive as read/write. Or you could manually umount the drive and then mount it with the correct rw mode. Check your man pages for the exact syntax to use.

    • Chuck H
      July 20, 2013 at 3:00 am

      Thanks Bruce and everyone else that responded. The problem finally corrected itself.

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