How TimeMachine works in simple terms, and why you should NEVER….go into it and start deleting files!!!

Someone just asked me “how do I delete files from my TimeMachine drive to make more room?”

The answer is, you don’t.

YOU don’t do anything – TM does it. The issue is that if you go willy-nilly deleting things off the backup, you’ll mess it up. Most of the things on a TM backup are not, er… “things”. That is, on your Mac (not TM) drive a file icon represents a real file, with data in it. Toss it in the trash, and you’ve lost the data. On the TM drive, most of those icons are “symbolic links” pointing to the data, which is elsewhere on the TM drive. The problem is, without some skill, you have no way of knowing which is which. (Why are they symbolic links? Because for most TM backups, most of the data itself (on your source drive) has not changed, so there’s no point in wasting TM disk space backing it up. The symbolic links maintain the fiction that each day’s or hour’s backup on a TM drive looks like your full hard drive. Most of it really only points (via symbolic links) back to the very first TM backup on the drive, which -does- contain real data.

To set -what- you want backed up, you use the TM preference pane, which work “backwards.” That is, it defaults to backing up everything. So you tell it what you want excluded from the backup.

For example, I use SuperDuper to maintain external drives with an exact copy of my main boot drive, and my work drives. I do that manually, because I have to physically mount the destination drives. Therefore I don’t need TM to back up that main drive. (Besides, it has over 2 million files on it and is nearly 500 GB in size.)

I wan’t going to admit this until a few more weeks went by, (because of problems I’ve had with TM in the past, backing up that huge main drive of mine) but I’m trying a TM backup myself. However, this time (and unlike most users) I have completely excluded my main drive (and thus everything on it) and am using TM only to back up my “Work” drive. This drive has considerably less stuff on it, (and none of it is intricate, such as the operating system) so I’m hoping that my previous issues with TM will be history.

So: what is supposed to happen when the drive fills up, is that TM will go back thru the history and consolidate. It will replace actual data files from the original backup where in subsequent ones a NON-symbolic link was used (that is, the file really did change on the source drive) and then remove older symbolic links (they are tiny, but not they do have some size on the drive.) It will clean that all up and make it look as though your TM copies no longer start in (for example) January 2008, but begin in (or “now only go back to”) April 2009.

What may be confusing to some is that each individual backup looks as if it’s a full backup of your whole hard drive. It is not. It just looks that way, so that you can say “Oh my gosh, I need to back to last Tuesday’s copy of my accounting records and start over” and it will simple and logical to do that.

Other backup methods don’t make retrieval of old information that simple; don’t offer such a pleasant face on it.

Does that help?

Tracy