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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