In betas of Mac OS X 10.5 Leopard, Apple introduced a feature that enabled Time Machine backup to a USB hard drive attached to an AirPort Extreme Base Station. However, by the time Leopard shipped, Apple had pulled the feature (see “,” 7 April 2008), focusing networked Time Machine backups on the Time Capsule (now called AirPort Time Capsule). On subsequent updates to Mac OS X, we checked to see if the feature had returned, and , but Apple steadfastly maintained that backing up to an AirPort Disk was unsupported, and eventually we stopped looking.
In fact, Apple still makes that statement in “,” saying clearly:
“Time Machine is not supported with USB hard drives that are connected to an AirPort Express or AirPort Extreme device.”
But a release note for the recent “” (21 November 2013) caught my eye while I was editing our coverage (this update applies only to the newest 802.11ac base stations released in June 2013). :
“Addresses an issue that prevents a USB hard drive connected to an AirPort Extreme from appearing as an available disk in Time Machine”
And once I went looking for confirmation, I found it in Apple’s “” document, which says:
“Time Machine can only back up to an external drive connected to an AirPort Extreme 802.11ac base station. Time Machine cannot back up data to external drives connected to other AirPort Extreme base station models”
That’s in complete contradiction to the 5-year-old prohibition against pointing Time Machine at an AirPort Disk, and the change has gone almost entirely unnoticed since June 2013, despite being prominently mentioned on for the new AirPort Extreme Base Station. Apparently, Mac users really don’t usually read the manual.
In fact, someone did notice — the late Time Machine expert James “Pondini” Pond in June; alas,  at the age of 70 and will be missed.
What’s most compelling about backing up to an AirPort Disk, as opposed to an AirPort Time Capsule, is that you’re not combining two entirely unrelated pieces of hardware in one package. If the AirPort Time Capsule’s hard drive fails, it’s difficult to replace, and if you want to upgrade the wireless gateway side of the equation, there’s no easy way to bring your backups over to the new device. With an AirPort Extreme Base Station and attached USB hard drive, you gain more flexibility.
I don’t have one of the new 802.11ac AirPort Extreme Base Stations to test the return of Time Machine backups to AirPort Disks, and everything I’ve read indicates that it works only with the newest hardware, not any of the older, flatter base stations. Nevertheless, if you have the necessary hardware and curiosity, I’d encourage you to hook up a USB hard drive and see if Time Machine backs up to it reliably (since 10.8 Mountain Lion, Time Machine has allowed you to back up to multiple destinations, alternating between the different destinations). Let me know how it works out in the comments!