Mac – What Are The Correct/Default Volume/Folder/File Permissions For A Time Machine Image

permissiontime-machine

Months ago, I created a disk image on a large external drive to house my Time Machine backups.

Yesterday…something went sideways with the permissions of my entire installation.

Included in this permissionpocalypse, is somehow my Time Machine dmg.

I am, now, unable to backup to Time Machine because the disk image is considered read only by 'system' and Finder informs me that I do not have permission to change the permissions of the image.

When I attempt to Verify the image in DiskUtility, I am told that: 'The image does not have a checksum so cannot be verified.'

So…before I investigate ways to super user sidestep my lack of permission to change TM dmg permissions, I figure I should probably know what those desired permissions are supposed to be.

Thank you.

EDIT: I'm using Mojave 10.14.6 (18G2022)

Best Answer

Time Machine sparsebundles use a very customized set of ACLs on the image file itself and the enclosing folders. Here's an example from a image created by a TM server for a Mac laptop (Qwe's laptop).

>ls -lae
total 16
drwxrwx---+  5 Ben  staff   170 14 Jul  2018 .
 0: group:com.apple.access_backup allow list,add_file,search,add_subdirectory,delete_child
 1: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
 2: user:qwe allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
drwxrwxr-x@ 32 Ben  staff  1156 29 Nov 11:59 ..
-rw-r--r--@  1 qwe  staff  6148  2 Sep  2015 .DS_Store
 0: user:_spotlight inherited allow read,execute
 1: user:qwe inherited allow read,write,execute,append,readattr,writeattr,readextattr,writeextattr,readsecurity
-rw-r--r--+  1 Ben  staff     0 21 Jul  2016 .com.apple.timemachine.supported
 0: user:_spotlight inherited allow read,execute
 1: user:qwe inherited allow read,write,execute,append,readattr,writeattr,readextattr,writeextattr,readsecurity
drwx------+ 11 qwe  staff   374 10 Dec 19:05 Qwe's MacBook Air.sparsebundle
 0: user:_spotlight inherited allow list,search,file_inherit,directory_inherit
 1: user:qwe inherited allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit

.
It's generally better to backup directly to an HFS+ format drive directly connected to your Mac, without using an image, if you can. Using disk images is reserved for network volumes and disk formatted to other file system types.

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