How to manage the dance of disk images, Time Machine, and excessively large backups

Disk Utility lets you create disk images that are virtual volumes. They’re stored as a file on a disk, but can be mounted and managed just like a directly connected or network-mounted volume. You can even use Disk Utility to create an encrypted version using a popup menu when you select File > New > New Image. (Pick 256-bit AES, as there’s no reason to pick the older format; read more in this older Mac 911 column.)

Problem is, Time Machine won’t back up the contents of disk images. They appear in the Time Machine preference pane when you click Options as grayed-out items in the Exclude These Items from Backups list.

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In order to back them up, Time Machine has to copy the disk image file. If you make any change to the contents of a disk image, the entire file winds up copied to Time Machine. That takes system resources and, for any sufficiently large disk image, also causes older versions of files to get dumped more rapidly to keep newer updates that include the changed disk image.

The way around this is to use sparse bundle disk images. These combine two excellent disk image properties: sparse and bundle. The sparse portion means that you can set any arbitrary large size to your disk image, like 100GB, but the space on your physical or network volume is only as much as the current data stored on the disk image.

The bundle part refers to the hidden structure of the disk image file. “Classic” disk images, which are created using the read/write option in Disk Utility for a new disk image, reserve as much space for the file as the size you define the disk image as and store data in that disk image as if it’s one large file.

With bundles, the disk image is a package that looks like a file, but which contains many smaller files, called bands. Each band is either 1, 2, 4, or 8MB depending on the defined full size of the disk image.

When changes are made to a sparse bundle disk image’s contents when it’s mounted, only the necessary bands change. Time Machine and other backup software that correctly understands macOS’s package format only back up changed bands.

This works with encrypted images, too. Time Machine doesn’t need to know anything about the contents of the disk image to back up the bands.

This Mac 911 article is in response to a question submitted by Macworld reader Steve.

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