This bug is actually kind of interesting, although it could cause some more significant problems in some cases.
I was using Disk Utility to do a free space wipe of one of my drives. What happens when this occurs is Disk Utility creates a new file that’s exactly the size of the free space on the drive, then overwrites the file however many times is specified in the wipe options. That would be either once, seven times or whatever the highest, most secure option is (I forget at the moment).
So that’s what I was doing when Time Machine started up. Time Machine noticed this “new” file that Disk Utility had created, and decided it needed to be backed up…all sixteen gigabytes of it. Suddenly, the backup was sixteen gigabytes larger than before, meaning my Time Machine drive did not have enough space. Older backups needed to be deleted to make room. I ended up losing two weeks of my oldest backups. This is not a huge deal in this case, but it is entirely possible that someone will encounter this bug at some point and experience far greater problems with it. It all depends on how much of a shortfall is caused by the wipe file. If the wipe file is, say, 75 gigabytes larger than the free space on the Time Machine drive, then you’re going to lose 75 gigs of backups just to make room for a gigantic file full of gibberish or zeros. Thrilling, huh? Even if you’re lucky like I was, with a wipe file “small” enough to not cause serious mayhem on the backup drive, you’ll still have a huge, utterly useless file sitting there, potentially for quite a long time, with no easy way to get rid of it. Why? Because it’s an invisible file, so you won’t be able to see it in Time Machine. [footnote!]
I wonder if this is fixed in Snow Leopard. I am a luddite, still using Leopard, 10.5.8.
The easy workaround, of course, is to turn off Time Machine before doing such a wipe. Obviously it is ridiculous that people should have to remember something like that, but that’s the way it goes in the computer business.
On the plus side, at some point in the future when Time Machine needs more space on my drive, and has reached the point where the September 18th backup is the oldest, I’m going to be getting a fairly large chunk back all at once. I wonder how long it will be? [footnote–not long, I guess! :P]
Footnote: I was able to get the wipe file off the backup drive and reclaim that space. It proved to be about 15 gigabytes, not 16 like I thought. How did I get rid of it?
The file itself was located a few levels deep in an invisible folder named “.Temporary Items” on the drive I was cleaning. In order to show this folder, I needed to make invisible items visible. Moreover, I needed to do this in a way that I could see the invisible files in Time Machine. There’s a utility called Onyx which can do this. It has a checkbox to unhide invisible items in the Finder, which also makes them visible in Time Machine (as I discovered, to my delight).
Once those invisible items are visible, I started Time Machine and located the backup where the huge file appeared. It was the first backup after I started the free space wipe. Finder view options needed to be set to “show all folder sizes”, so I could check the sizes of the folders I was looking at. Basically, I kept opening folders until I found the very large file that was causing the problem–I think it was two or three levels deep. It had some generic, technical sounding name (I forgot to note it down before I removed it), but was clearly identifiable it by its size, which was equal to the amount of free space on the drive from before. I selected that file, then went up to the little actions menu button in the title bar of that Finder window (it’s the button with the little gear icon in it). One of the options was to remove all backups of that file. It asked if I was sure and requested my password to confirm. After that, I had the space back on my backup drive, all fifteen gigabytes of it. Sadly, though, there is no way to recover the old backups which had been deleted to make room for it. :(
Disclaimer: Just because I am describing this here doesn’t mean I am recommending this procedure. Anything you try is at your own risk. In particular, do not use the Finder to delete individual files off of a Time Machine drive, because it will not work. I am not responsible for the actions of people who try this who don’t know what they are doing. I’m mostly putting this description up because I am sort of a compulsive explainer, and can’t really help myself. ;)