Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you.

Can anyone tell me why?

Your backups are corrupted.

I am afraid to start a new back up if I have to erase everything first.

Then, you can copy all of the data on the Time Capsule to a USB drive before you click to start a new backup. But....because the backups are corrupted, they will be of no use to you.

Using Sierra? Apparently you need to regularly back up the backup and restore it every time Time Machine corrupts it, or find other backup software besides Time Machine. Or figure out what in your local network might be "causing" the corruption.

My guess is that Sierra is somehow lacking in its network error recovery, but I really don't know. We have two MacBook Pros in our home network (which is now using a brand new Airport Express for its router), and each one "developed" the problem when Sierra was introduced. Actually, the second one is a brand new MacBook Pro that came with Sierra installed. It replaced an older Macbook that never, ever encountered this problem in our network. The other is a mid-2015 MacBook Pro that did not run into this issue until upgrading to Sierra. Time Machine currently fails every few days, although the frequency has varied over time since installing Sierra.

No doubt. But since OS versions prior to Sierra did not require a "perfect" home network to function, and many home networks are less than perfect, and Sierra doesn't provide any user-level tools of which I am aware to diagnose such "imperfections", and since Apple has been removing diagnostic capabilities from its network devices, I can't help but feel that Apple should be addressing this issue, even if it only shows up in a minority of networks.

The most likely suspect in my network is the external Apple Thunderbolt Ethernet adapter that I have to use with my MacBook Pro, which is a recent model that no longer contains an internal adapter. It is unreliable, and I noticed that Apple also seems to have ceased to sell them, relying on other vendors instead. The adapter that I used for the first year or so failed outright, and I am using a spare that I bought at the same time. Just now it is showing the first visible symptoms of developing an intermittent connection, after a couple of months of use, and presumably it will also fail outright within another year. I am considering going to a USB Ethernet adapter, because it has a more solid connector (USB instead of Thunderbolt) and it doesn't place the RJ-45 connector inline & close to the Mac, where it is bound to cause problems.

If that is the case, it would seem that "my" problem is Apple's problematic hardware. Under those circumstances, I would still very much appreciate it if Sierra TIme Machine would do proper error recovery in my imperfect network, the way earlier versions did, even when confronted with a poorly-designed Apple Ethernet adapter.

If that is not the case then nearly all of the rest of our home network is Apple devices as well, and my feelings remain the same.

For me, it started when I upgraded my MacBook Pro to Sierra. Another, older MacBook on the same network but not on Sierra (and used by someone else) was unaffected. When the other Mac was replaced with a new MacBook Pro pre-installed with Sierra, it developed this same problem.

Up until now I have assumed that this is environmental, since most Sierra users don't have the problem (and I am assuming that many of them are running Time Machine using a networked storage device). Another possibility to explore, however, is that it could be related to using Time Machine to back up documents that were migrated to iCloud when configuring Sierra. For my situation, both upgrades to Sierra were accompanied by migrating the Documents folder to iCloud, so either Sierra itself or the Sierra implementation of Documents-in-iCloud could be the trigger.

This just happened to me last night after running Time Machine without issues for a very long time (years?). I have my backups going to a relatively new (since January) 4TB seagate backup drive that is connected via USB to my synology chassis. This has been working fine up until now. I am loathe to lose all my incremental backups even though it's only been a month. And from some of the comments, once started, this appears to be an on-going issue.

I came across this bit of information:http://jd-powered.net/notes/fixing-your-time-machine-backup but a not confident to try it. Has anyone else tried it or something like it? I am not very technical and am not confident that I could execute it successfully. And it's a bit old so not sure if would still work quite as stated.

Thanks

You can usually make your backups look as though nothing has happened, by following a procedure such as that one. Whether the data itself is actually corrupted or not is another matter. Flagging it as not corrupt does not fix any corruption that may have occurred.

I've been trying to track down this same issue. I'll admit I haven't read through this entire thread, but from what I've seen it appears to be a bug with Sierra for Macs that are backing up in certain situations (I've only seen it so far for Macs that are backing up to network drives via Ethernet). I have filed a bug report with Apple, so they're aware of the issue.

It does it to me using Mavericks (Yosemite gave me WiFi and BT issues that Apple was, ummmm, disinterested in fixing, so I went back to Mavericks, which had zero issues with those cutting edge technologies). The Mac in question was never upgraded to Yosemite.

I only have an Apple Time Capsule hardwired to the Mac. Hardly seems like a config they wouldn't have envisioned.

Apple is very aware of this problem.

Apple is very aware of this problem.

And has been for more than 6 years......so you will not likely see a solution from Apple in the near future. See the link below for more history and information on this issue.

http://pondini.org/TM/C13.html

I encountered this problem after upgrading to Sierra, like most people commenting here.

It recurs every two months or so. My Airport firmware is up to date, and I have no other network related problems.

For my setup, at least, I've finally solved it.

I have 4 different machines backing up to same TC.

No problems for several years, then began to have troubles with 10.11 and persisted into Sierra

Only one iMac would get this error message, the other 3 were fine.

I even tried reformatting the HD in the Time Capsule with no improvement.

I then did clean reinstall of Sierra on the involved computer, now no problem for a month and counting!

After the clean OS install, all other programs were clean installed also, minus ClamX.

It seems that the ClamX Sentry may have been the culprit.

pjleeson wrote:

It seems that the ClamX Sentry may have been the culprit.

Interesting conclusion, but one that nobody else has reported. It would be helpful if you could verify this and then report it by opening a ClamXav help desk ticket. Provide as much detail as you can as well as link to this lengthy discussion. I suspect you will be asked to run the built-in diagnostics in order to gather complete information on your setup.

You can always remove it after, if necessary, by dragging the application to the Trash Can and within a few seconds you'll get an alert offering to remove the remainder of the installation.

I have this problem too and do not have ClamX so I don't think it has anything to with it. Probably reinstalling got rid of something that was causing the problem, or you will get it again in due time

I just got this message on a late-2013 13" MacBook Pro Retina backing up to a 2TB Air Capsule. Didn't know what to do so it's now doing the painfully slow new backup.

But: What is the worst thing an automated backup program can do? Destroy all your backups without offering any recovery option, in the name of "to improve reliability". And that's what this program does. Get to work on this Apple. It's not like the Time Capsule is cheap. Earn your margins with reliability and functionality!

Hi James,

I reported this as a bug to Apple and they seem to be working on it, since they have asked for diagnostic info and sysdiagnose data. Until it has been solved, I turned off my TC because I don't want to lose my backup. In the meantime I backup using Carbon Copy Cloner and Arq (cloud backup)