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Why Leopard’s Time Machine Doesn’t Support AirPort Disks.

Why Leopard's Time Machine Doesn't Support AirPort Disks.
Daniel Eran Dilger
In the previous article looking at what’s new in Leopard, I identified three missing features that some had expected to see. The most obvious was using Time Machine to back up to shared AirPort drives. Mention of the feature on Apple’s site was removed, indicating that the company wasn’t bluffing when it warned that “features are subject to change.” So where did it go, and why?


When I wrote the article, I didn’t really know. I suggested the problem was likely either related to WebDAV support or to feasibility problems in using disks over WiFi. The former sounded like something that could be fixed, while the latter might result in the whole idea being abandoned.

Reader Alex Curylo wrote:

This was explained on one of the developer lists a couple weeks back. The problem is that integrity cannot be guaranteed — the AirPort acknowledges receipt of the data before it’s actually written, and if power is interrupted, the disk disconnected, yadayadayada in the window between the Airport acknowledging receipt and the data actually getting written out to disk, it’s gone forever with no way to recover it or even realize it’s gone missing.

That, Apple felt, is a big enough problem to disable the feature for initial release. Quite reasonably so, I think most of us can get behind.

Unfortunately, to fix this issue requires firmware upgrades on the AirPort side so that there’s some way it can send back “Yes, your data is now
written and safe on disk, I guarantee it” rather than simply “Yep, gotcher data, right here in RAM, looks good, trust me.” So that’ll take, well, however long it takes, and if/when that happens wireless Time Machine backup will be back, no doubt.

That sounds like good news for Mac users who bought an AirPort Extreme with the idea of using it for Time Machine backups. Support may come as soon as 10.5.1, which is noted to address issues with both Time Machine and AirPort.

AppleInsider | First builds of Mac OS X 10.5.1 pack over two dozen fixes
A base station firmware update is also due, which will likely link the AirPort Extreme’s “wide-area hostname” and “advertise globally using Bonjour” features with the Back to My Mac marketing name used in Leopard and automate integration with .Mac authentication.

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What do you think? I really like to hear from readers. Comment in the Forum or email me with your ideas.

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20 comments

1 Brau { 11.09.07 at 3:17 am }

the company wasn’t bluffing when it warned that “features are subject to change.”

Funny, I don’t recall reading that when I convinced two people to buy one for that express purpose. It was NOT made clear. I sincerely hope Apple will fix this soon because if they don’t then the line between good business and false advertising/fraud feels like it has been crossed. How many “missing features” would you tolerate if you ordered a TV, brought it home, and found it did not work as advertised? Apple better make good on their promise.

2 John Muir { 11.09.07 at 6:01 am }

Here’s to hoping. My AEBS feels like a function update.

3 HG { 11.09.07 at 10:34 am }

An Apple source of mine (not high level in engineering though) said the reason for the missing Air Disk feature was because the Air Disk engineers use the SMB protocol and the Time Machine engineers use AFP.

They’re sorting it out.

4 danieleran { 11.09.07 at 4:29 pm }

The passing mention of AirPort with Time Machine was more subtle than the disclaimer on every page of the Leopard preview saying that features were subject to change.

There’s a big difference between being disappointed and being defrauded.

It’s hard to say you bought Leopard and found it didn’t work as advertised when the idea of AirPort/TM use was widely advertised as not being available in 10.5 before it went on sale.

I believe AirDisk actually uses WebDAV, just like .Mac’s iDisk, rather than either SMB or AFP. SMB is Microsoft’s proprietary Windows sharing protocol, and use of it is subject to patent retaliation from Microsoft. WebDAV is an open protocol, so Apple is working to move things in that direction.

AFP is a session based protocol that doesn’t like to be disconnected; WebDAV is essentially a two way web server, so it doesn’t mind being disconnected in theory. It is better suited to Internet file sharing (which is why it’s used on .Mac and over WiFi.)

5 Brau { 11.09.07 at 10:59 pm }

Just to be clear here:
I don’t believe for a second that Apple has any intent to defraud. I think issues have cropped up in regard to security and reliability so Apple has pulled it, in keeping with their quality control policies. What I am most interested in is their lack of an explanation, which seems to suggest they don’t know for sure at this time whether they can make good on this feature. It would seem to be a safe bet it would be added in the near future if it is only a soft/firmware issue. If it is a hardware issue, then those who bought an Airport Extreme during the time Apple was advertising this feature should qualify for a full refund or credit.

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8 Perché TimeMachine non supporta i dischi Airport { 11.12.07 at 12:43 pm }

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9 MACNOTES.DE » Notizen vom 13. November { 11.13.07 at 3:01 am }

[...] noch vorhanden. In der Final sucht man bislang vergeblich nach diesem Feature. Das Online-Magazin RoughlyDrafted weiss offensichtlich warum. Sichert man seine Daten mittels Time Machine auf einer an die AirPort [...]

10 Technically Sound » Blog Archive » Risky Data Ops { 11.13.07 at 3:17 am }

[...] RD: Leopard’s Time Machine Not Supporting Airport Disks is a Good Thing This was explained on one of the developer lists a couple weeks back. The problem is that integrity cannot be guaranteed — the AirPort acknowledges receipt of the data before it’s actually written, and if power is interrupted, the disk disconnected, yadayadayada in the window between the Airport acknowledging receipt and the data actually getting written out to disk, it’s gone forever with no way to recover it or even realize it’s gone missing. [...]

11 iMac Blog » Blog Archive » Time Machine and Wireless Network Drives { 11.13.07 at 7:37 am }

[...] It could have been a nice option for an Airport Station 802.11n plugged to an USB external HD. Roughlydrafted reports that for security reason, Apple might have chosen not to let this option activated. Indeed, [...]

12 Apple Blog » Blog Archive » Time Machine and Wireless Network Drives { 11.13.07 at 7:43 am }

[...] It could have been a nice option for an Airport Station 802.11n plugged to an USB external HD. Roughlydrafted reports that for security reason, Apple might have chosen not to let this option activated. Indeed, [...]

13 Using time machine with a NAS » Plunge into Mac { 11.20.07 at 2:23 pm }

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16 Time Capsule and Its Associated Rage Factor { 01.17.08 at 12:08 pm }

[...] pulled from Leopard before the new OS shipped because of reliability issues. (Roughly Drafted had a technical explanation for this back on 2007-11-08; Joe Kissell discussed overall Time Machine problems, including this one, in [...]

17 Macinsoft - Time Capsule and Its Associated Rage Factor { 01.26.08 at 3:35 pm }

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20 Apple Time Capsule, No Thanks Apple! { 04.23.09 at 2:39 am }

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