This setup assumes the following: Mac Pro with connected FW800 external drive and MacBook Pro laptop. Mac Pro backs up locally along with the MBP storing a sparsebundle over the network onto the same disk.
First, turn off Time Machine for the device associated with the sparsebundle you are changing. At a high level, the steps are:
- Create a new user on the Mac Pro called backup.
- Recursively change the permissions on the sparsebundle to include backup.
- Check whether the mount works correctly from the MBP.
- Turn Time Machine back on.
Create a new user on the Mac Pro called backup
The first order of business is to create a new user from the Account Preferences screen. I created one with the name of Time Machine and used the username of backup. I also selected the option to make this account a "sharing-only" type, because I am not planning on using the account to login.
Recursively change the permissions on the sparsebundle to include "backup".
This is the tricky part. Yes, you can chown -R backup *sparsebundle on the sparsebundle you are working with, but then you lose the ability to look at it with your personal user account. So, I used the ACL facilities of chmod to add the new backup user into the mix.
$ cd /Volumes
$ sudo find Bob\'s\ MacBook\ Pro.sparsebundle -type d -exec chmod +a \
"backup allow read,write,list,add_file,search,add_subdirectory,readattr,writeattr,readextattr,writeextattr,readsecurity" {} \;
$ sudo find Bob\'s\ MacBook\ Pro.sparsebundle -type f -exec chmod +a \
"backup allow read,write,append,readattr,writeattr,readextattr,writeextattr,readsecurity" {} \;Mount the network share using the backup user and subsequently, try to mount the sparsebundle that is stored within. If you see a folder icon with a red circle to the lower right, the permissions are messed up. Go back to the Mac Pro and use ls -le to check out what the ACLs are on the folder.
If you don't know how to flip over from your personal account to backup on the MBP, you can open up Keychain and delete the password from the System keychain. This will force Finder or Time Machine to re-authenticate.
Turn Time Machine back on.
If you can mount via Folder, Time Machine will work. Turn it on and enjoy an incremental backup using your new user.
[robg adds: I haven't tested this one.]

