One feature I dislike about OS X is the Time Machine local backup function (also known as MobileBackups). Mostly, I dislike that the average user has no control over it. Like many, I’ve sent feedback requests and bug reports, all of which have gone unanswered by Apple. It was added in OS X 10.7 Lion.

For those that don’t know, feel free to search about the “feature” – but basically, when a portable Mac is not connected to a Time Machine drive or Time Capsule, it will start backing up changes since the last backup locally… right on your startup drive. And you never authorized Apple to do that. They just decided they should do it, not considering the massive security flaws with leaving backups (which you don’t even know exist) on your startup drive, especially when the drive isn’t encrypted.

A good example would be if you emptied the trash on an item… surprise! I can probably recover it using Time Machine local backups. Moreover, TM local backups eat up free hard disk space, though Apple claims it will start deleting local backups if the drive gets full. Of course, a user then has no clue how much disk space they actually have left. I presume this feature applies to files deleted by Secure Empty Trash as well, though I haven’t tested… I don’t want the feature on any of my machines to begin with.

Needless to say, I don’t like the feature. It should have an off switch. Users should be made aware that the feature is being silently enabled. For the reasons above, I consider it the biggest security hole on Mac today as a result. To disable it, fire up Terminal and type sudo tmutil disablelocal and hit enter.

That said, this post is to warn you that when you get a new Mac, Migration Assistant will not carry over this setting. Even though it will carry over all your other Time Machine settings, MobileBackups is still left turned on. You’ll need to disable it once again each time you get a new Mac. I would also check after each OS X update, especially major ones.

P.S. Time Machine local backups are not enabled by default on any desktop Mac model. Not sure why, since they would “benefit” more with their 1 TB+ hard disks. You can enable it via Terminal however.