The bMighty Blog -- Apple

Yesterday, one of the Leopard-based iMacs experience a strange Time Machine error — and that error is making me uneasy about the possibly buggy backup software.

This particular iMac is equipped with Mac OS X 10.5.2, and has been running Time Machine since February. The external hard drive that it's attached to is a 500GB drive, and is 100% dedicated to Time Machine. Since the Mac has a 250GB internal drive, that means plenty of space for backups. Right? Wrong.

The machine's user brought my attention to the error message that showed up mid-morning: "Unable to complete backup. An error occurred while creating the backup directory."

It turns out that the Time Machine volume had only about 230MB of free space. This should not have been a problem. According to Apple's Time Machine specs, old backups are supposed to be deleted to make room for new ones. As Apple says,

Backing up to a full disk. One day, no matter how large your backup drive is, it will run out of space. And Time Machine has an action plan. It alerts you that it will start deleting previous backups, oldest first. Before it deletes any backup, Time Machine copies files that might be needed to fully restore your disk for every remaining backup. (Moral of the story: The larger the drive, the farther back in time you can back up.)

The user, however, never saw a warning about deletions of old backups. Instead, Time Machine just halted with an error message. Apple's apparent remedy: Get another backup drive! This is what the support page says:

When you first set up Time Machine, it performs an initial backup of your entire computer. Subsequent backups find and save new items and items that have changed, so the backups become smaller. Also, as your backup disk fills up, Time Machine deletes older backups to make room for new ones. You may be able to use Time Machine for a long time before running out of space on your backup disk.

If you do run out of space, the best thing to do is to attach a new backup disk. After you attach the new disk, open Time Machine preferences and click Change Disk to choose it as your Time Machine backup disk.

There's clearly a disconnect between the expected behavior (i.e., Time Machine will erase old backups to make room for new ones) and the real behavior (i.e., when Time Machine fills the disk, you need a new one). Not good.

On the short term, I manually went into Time Machine and deleted old backups, to free up about 60GB of space. Backups are now proceeded properly. However, the deletion of old backups is supposed to be automatic — that's part of the benefit.

This situation is causing me to reevaluate the suitability of Time Machine as a backup system for business Macs. It should cause you to reevaluate as well.

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