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Is there any technical limitation or engineering reason that could possibly be behind the inability to RELIABLY use a connected USB drive with Airport Extreme for Time Machine?

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We generally delete questions that simply ask people to justify why Apple has done X, Y, or Z - especially those that are ranty. Would you try editing this down to the barest facts and see if people have interest in reopening it? –  bmike Aug 3 '12 at 18:35
    
Here's an example of a more "how does X work" or "how do I work around restriction Y" that tend to work better for our site looking for authoritative answers. apple.stackexchange.com/posts/59033/revisions –  bmike Aug 3 '12 at 18:38
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Perhaps re-opening as "how can I use TM with a NAS or USB-AE disk" - as it actually is possible with minimal effort. –  stuffe Aug 3 '12 at 18:57
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Note: For a backup solution, I would strongly recommend only using a supported configuration. What good is a broken backup? –  Thorbjørn Ravn Andersen Aug 7 '12 at 23:31
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@ThorbjørnRavnAndersen: yes I totally agree. I'm just trying to understand what's going on here that would prevent this from being a supported configuration. No matter who edits my question, and no matter who much 'karma' they have, I'm not asking how to get it working. (Now if someone has a bulletproof workaround, I might be intrigued, but that's not what I'm asking about.) –  iconoclast Aug 8 '12 at 0:14

4 Answers 4

up vote 2 down vote accepted

Not a complete answer, but this document might help: Time Machine Server Requirements

To be listed as a backup destination, the AFP server must advertise it with specific features and flags turned on. Airport Extreme storage doesn't show up because it's missing something from that list, but the question is what?

I found a reference to the Extreme's AFP implementation lacking some caching capabilities (link) but I have no idea if that's true. It is plausible that it doesn't have the storage required for the replay cache.

It's also plausible that it does have all these features but Apple chose simply not to enable the Time Machine flag.

Note that the newest generation (the big ones with 802.11ac) does now officially support Time Machine, so this doesn't seem to be an issue anymore. That's probably because they're now almost identical to the current Time Capsule inside.

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So it sounds like (if I'm understanding all this) it boils down to this: they didn't give Airport Extreme a complete AFP server (perhaps to keep the hardware cheaper and still get decent performance?) and as a side-effect it couldn't reliably work for Time Machine. –  iconoclast Aug 8 '12 at 19:12
    
This seems to be the most complete answer so far, perhaps the only remotely complete answer. I'm not sure why I got so much flack for even asking the question in the first place. You've proved that a pretty-much authoritative answer can be given (it came directly from Apple, after all) as to why this doesn't work. –  iconoclast Aug 8 '12 at 19:20
    
Yeah, I'd agree with that estimation. It's hard to figure out exactly what their motivation was, but they did advertise Time Machine on Airport Extreme before it was released. Maybe they pulled it for technical reasons, maybe they pulled it to prevent the Extreme from eating up Time Capsule's profits. The Extreme has been around for years, you'd think they'd have overcome any technical limitations by now. –  gabedwrds Aug 8 '12 at 22:37
    
Yes, one can definitely speculate on motivations--it's not hard to see what those might be--but I have a pretty good answer (from you) on the technical side of things. Perhaps to the chagrin of stackexchange naysayers who threatened to close this as "not a real question" ;) –  iconoclast Aug 9 '12 at 15:00

I have had this working fine, just by initialising a backup over local USB, then moving the drive to the AE, and selecting a the same drive over the network location. All worked fine.

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how long have you had it running this way? –  iconoclast Aug 7 '12 at 21:37

I've also heard it's unsafe, with the explanation of journaling, the "log book" for the hard drive. This journal contains the actions an operating system is "about to do", so the operating system knows what it was doing in a recovery after a write failure (for example, because of power loss). It's easy for a backup over a network share to be interrupted, for example, when the network temporarily falls away. Since a lot of caching is involved in network shares, including caching of the disk journal, you can't always be sure that the remote journal is updated, causing the physical on-disk journal to be out of sync with what the operating system thinks is in the journal.

All of this said, though, I still don't completely understand why it's not possible. Time Machine handles interruptions quite well as far as I know: until a backup is completed, it is in a separate place that doesn't confuse the software if it goes wrong. I.e. maybe the operating system can't fully recover all write problems but at least this can't mess up a backup. Only at the very end of the backup is the backup moved into place with a single, atomic operation. In my mind, the above explanation is a hint, but it isn't a complete answer...

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Yeah I agree it doesn't make sense. If Time Capsule can do it, there doesn't seem to be any technical reason Apple couldn't allow it with external drives on an Airport Express. Unless it has something to do with their choice of drives, or problems with the (slow) speed of USB. –  iconoclast Aug 8 '12 at 0:11
    
(BTW I wasn't the one who downgraded your answer.) –  iconoclast Aug 8 '12 at 0:12

There's no technical limitation. You simply need to plug the disk into your Mac and format it as "Mac OSX Extended (Journaled)", however, it's been reported to be unreliable.

Airport Extreme + USB Drive vs Time Capsule

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Yes, and because it is unreliable, it's a bad idea and something that most people should rule out. So I'm wondering if there's some technical limitation that makes that the case, or is Apple just making it the case? –  iconoclast Aug 7 '12 at 21:27

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