Time Machine doing

backupmac-osxtime-machine

Sometimes OS X tells me that it's "Preparing backup…" for an enormous amount of time. The time it takes seems to have no correlation to the amount of data to be backed up. Same for the "Finishing backup…" stage.

The question: what is OS X doing when it's "Preparing backup…" and "Finishing backup…"?

I know it's figuring out what to backup and then recording the changes, but surely the time this takes should be proportional to the amount of data that changed. There's plenty of space disk space (so I assume it's not deleting old backups). I'm using a TimeCapsule if that makes any difference. Nothing is going wrong with it, I'm just curious.

Best Answer

Here is an answer from another forum (original poster unknown to me):

The "preparing" stage in Time Machine is usually fast, because Time Machine uses a log of file system activities to know where to find changes that need to be backed up.

But there are things you can do to make that log untrustworthy. When Time Machine discovers the log MIGHT not be valid, it does a full scan of your file system to determine what really needs to be backed up. This can take a while. On my Powerbook (and with a firewire external Time Machine drive) it can result in a "preparing" stage that lasts 20 minutes. If you let Time Machine continue, it will eventually figure out what really needs to be added to the backup disk and will continue. This, of course, is better than just gambling the log is correct or just backing up EVERYTHING again even though only a few files might actually have changed (which would take much longer and consume more Time Machine disk space).

In these early days of Leopard installs, one of the most common things you can do that will make the log untrustworthy is booting from the Leopard install DVD. Each time you do that, the next time Time Machine runs it will discover that your hard drive MIGHT have been modified without the changes being reflected in the log. So it does the full scan -- a long "preparing" stage -- to insure it gets things right.

Related Topic