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In addition to root and nobody, there is another special unix account in OS X with some interesting properties: UID 99, Unknown User. Account 99 is treated differently by the operating system and has a couple of interesting properties that allow removable devices to be easily used between multiple machines, specifically:

  • Files owned by UID 99 will appear to be owned by the user who is accessing it. This applies to all accounts, except the root user.
  • Volumes that are mounted with the ignore ownership option will appear to have all files owned by UID 99.

Thanks to this feature, if you take your iPod or external hard disk from one machine to another, your account on machine 2 will be able to read and write to the device even if your UID doesn’t match your account on machine 1.

Sometimes, however, this isn’t what you want. Say you have a drive that you use with only one machine, and that machine is used by several people. This might be a scenario where it would be desireable to enforce user specific file permissions.

You can do this for HFS+ formatted volumes. Just right click the volume and select Get Info. Then, under the ownership and permissions section, uncheck “ignore ownership on this volume”.

The first time you do this, you’ll notice that all files on the volume will be owned by the account you were logged in as at the time. You’ll be able to set file ownership and permissions from this point forward and OS X will enforce it.

User 99, Unknown –Link.