What file service should you use with network drives and Time Machine?

A network-attached storage device needs to use the proper file service with Apple’s Time Machine.

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Apple

What kind of networked volumes formatted with which methods can you use with Time Machine? A reader queried me on Twitter, trying to understand from their past experience and Apple’s current documentation, and I agree that it’s not very clear, because of Apple’s inclusive but non-specific language.

Apple used to favor the Apple Filing Protocol (AFP) file service for network drive sharing, local and remote. However, it’s deprecated AFP in favor of the Windows-compatible Server Message Block (SMB) format, which is what’s typically used with network-attached storage (NAS) devices and non-Apple Wi-Fi routers that allow USB-connected drives. (A NAS is typically a standalone disk-drive connected to a network via ethernet that act as a media and file server.)

Some people, including my Twitter correspondent, think that SMB is still unsupported, but Apple explicitly calls it out in its Sierra support document for Time Machine. However, Apple notes that not all SMB (or AFP) shared volumes work with Time Machine, and doesn’t provide details on how to determine this without mounting the volume and seeing if it shows up as an available Time Machine destination in macOS.

If you want to make sure before you buy, look for marketing promises made by the product’s maker and for forum posts by people trying to (and succeeding in) getting it to work. For example, Synology makes some of the most popular NAS systems, and advertises Time Machine support and has a detailed tutorial on setting up a Time Machine volume.

Many people have recommended a clever bypass in a variety of forums, which I’d recommend only as a secondary backup method. You can create a sparse bundle disk image on a networked volume. To attach this to Time Machine, first you mount the networked volume, and then you mount the disk image. The disk image can be selected as a target for Time Machine when formatted in one of Apple’s approved methods. (The preferred one is Mac OS Extended format (Journaled) format, but Time Machine also supports Mac OS Extended (Case-sensitive, Journaled).)

This isn’t as reliable as using a disk that Apple recognizes, which is why I suggest not using it as your principal backup method in case something goes awry.

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