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.
[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.
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?
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.
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.