Recently, I acquired a Synology DiskStation and wired up a nice comforting Time Machine-to-Synology-to-S3-to-Glacier backup data flow. But then I started to see “Time Machine couldn’t complete the backup” with something about “could not be accessed (error 21)”. Here’s how it got fixed.

Time Machine problem

[This piece placed here to attract search-engine attention and, with luck, help someone else dig out. If you’re feeling public-spirited, toss in a couple links for visibility’s sake.]

I was tempted to give up, but the thing was working fine for my wife, who however was not yet on Sierra; there was some Twitter rumbling about Sierra and Synology having relationship problems.

With the aid of Google and Twitter, I eventually did all these things, and finally data started flowing again. Perhaps they are not all necessary.

  1. After a tweeted squeal for help, Saul Tannenbaum (@stannenb) and Shazron Abdullah (@shazron) suggested installing the Synology beta, and connecting over SMB not AFP. Because SMB is the new shiny, amirite?

  2. Also per @stannenb, poked around in the Synology menus (Control Panel>File Services>Service Discovery) to enable “Bonjour Time Machine broadcast via SMB”. Uh huh.

  3. At some point before the backups started failing, I’d seen a message about a Time Machine verification problem; Googling that led me to Jonas’ Fixing your Time Machine backup, which talks you through mounting the ”sparse bundle” and using the fsck hammer. Last time I used fsck it was in a different millennium. Thanks, Jonas. Like he said, it ain’t fast, and the use of a wired network is indicated.

Of course, a civilian would have been dead in the water, and I should be mad at either Apple or Synology, or the pair of them for not playing nice. But at the moment, I’m so relieved to have this working that I’m not.


Comment feed for ongoing:Comments feed

From: Scott Johnson (Jan 31 2017, at 23:04)

I had seen a couple of these errors since I upgraded a few weeks ago, but I had not even considered it being a Synology issue. Thanks for sharing!


From: Trung Duc Tran (Feb 01 2017, at 03:25)

I spent hours^H^H^H^H days and nights attempting to get TimeMachine over network working and I failed. At first everything seemed smooth sailing. Then suddenly TimeMachine says the backup is corrupted / invalid / something wrong. The only thing I could do is to reset it. That takes days to redo full backup of 750GB of data on my MBP disk. Then a week later, same error, same thing. Backup by definition should be stupidly reliable. Any fiddling with it gives me little confidence I'll be able to restore data when I'll truly need it.

Now I am back with an external USB hard drive. It's really annoying to plug in the disk and have to remember to eject it before taking the laptop somewhere. But at least TM works reliably with this setup.


From: John Doty (Feb 02 2017, at 08:12)

Not to poke too much, but do your time machine restores work? I have seen terrible terrible stories...

Two relevant links are this twitter story of Time Machine not actually functioning ( and of course GitLab's recent database troubles. (


From: Trecento (Feb 08 2017, at 06:05)

Hi Tim,

Last night I spent some time reading about chipset failures for a range of products, including Synology. The de-sensationalized version is this: there's a vulnerable batch of Intel Atom chips[1], Synology products use them in their motherboards, and they die after 18 months of total uptime. (Except when they don't, but apparently it's an uncomfortably likely error.)

The failure mode seems to be total, rather than partial, however, you may want to consider whether data corruption on your Synology device is likely, and if such corrupt data is being carefully backed up on Amazon.

Best of luck. I've been reading for years, and really appreciate your work.

[1] Atom C2000 system-on-a-chip.


From: Earl (Apr 16 2017, at 23:13)

Hi Tim,

I have a very similar setup and you've only temporarily fixed it, if my experience is any measure. The errors will be back.

I tracked it down, at least on my macbooks, to this: Time Machine really really *really* doesn't like it if you interrupt a backup in the middle, eg by shutting the lid on your macbook. So one option is enable a no-sleep program while backups are going on. But a small glitch in your wifi will produce the same result.

I'm going to sound like a salesperson, but I'm not, just a really happy customer. The solution is software called Arq Backup. It will backup to a network mount (use smb), or directly to aws or google cloud. It does incremental backups, size capping, verification, all the nice stuff. And it's $50 a license. I've been using it for two macs in my house for over a year after hitting all the same problems and, in that time, no issues.

Good luck to everyone google sends here with busted Time Machine backups.


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January 31, 2017
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