Q: Time machine not working after upgrade to Sierra

Interesting. I can't get Time Machine to complete a backup to an external HD directly connected to my MacBook Pro. It'll either start and stall shortly afterwards, or today it spent the whole day "Preparing backup...". It also looks like the diskimages-helper process has been using up an entire core all day. (Seems my Finder isn't responding now either. I'm wondering if something went wrong during my install. Maybe I should try to reinstall).

I am seeing the same behavior after installing Sierra on my MacBook Pro.  Time Machine just hangs in the "preparing backup" phase for hours.  Activity Monitor shows process "backupd" continuously using a large portion of CPU.  My backup disk is hooked directly to my Airport Extreme.

Likewise, since upgrading to Sierra, my Time Machine shows preparing backup but stalls at 1.3mb of about 18gb. Let it run overnight but still doesn't get anywhere. I use a Seagate Backup Plus Slim 2tb external drive which worked perfectly fine before the upgrade. Loathe to re-format as it appears from discussion that doesn't help either.

Same problem here, it gets stuck at "Preparing backup..." mode. It's a 3 TB Apple TC with Airport base station and worked without problem on El Capitan.

The backup is working now, after 30 minutes of "Preparing backup...". The estimated time doesn't match normal time under El Capitan. In addition, the console log messages don't show the normal information it did under El Capitan, meaning HD and file progress. I hope Apple restores this as it's useful to see what it backed up.

Same issue. Time Machine working fine on El Capitan. Now spends ages in "Preparing backup...." backupd is taking 98% CPU and logd 30% at this stage. I have had a successful backup eventually after leaving to run overnight.

I went to the System Preferences Time Machine window and reselected my backup disk.  Now it seems to be backing up 20 GB, but at a snails pace compared to El Capitan.  After 12 hours it is half done and says it needs another 7 hours.

The current backup (first) is taking 3-4 times longer than El Capitan. It was 10-12 GB's per hour with El Capitan and this is 3-4 GB's per hour. Seems Apple like the keep reinventing the wheel without thoroughly testing it before release.

I moved my backup image to another external hard drive and it seemed to function normally - after the 4 hours to move over 400GB. I put the original hard drive on my Airport Extreme and the Airport Utility wasn't reporting a drive attached at all. I attached the other drive to the AE and it was visible. Maybe it's just that my drive is dying, or that the drive has issues *and* there are Sierra issues. Doesn't explain why the backup worked when I moved it though. I enabled encryption on the replacement drive and when I left this morning it was Preparing Backup.... Hopefully when I get back to it everything will be normal again.

I have the same problem. After 24 hours of "Preparing backup" I cancelled the backup, restarted my 2013 MacBook Air, and tried again. The 2nd time I let the Time Machine backup run for 30 hours. It never left the "Preparing backup" status, so I cancelled it, restarted my computer in the Safe mode, and ran the backup again. This time it only stayed in the "Preparing backup" status for 40 minutes, then, it actually started backing up. After another 40 minutes the backup finished successfully. So, the bottom line is, it will work in the Safe mode, but not in the "regular" mode. There was a Sierra 10.12.1 beta 1 released to the beta testers two days ago. Maybe when it's official it will resolve this problem.

Good luck with safe mode. I've never had good luck with it as it takes a long time and eats memory on the startup. As for the backup, two things I noticed with OS-X 10.12. First the console log no longer writes the information about the progress of the backup it did with 10.11.x. Second, the Time Machine widget no longer works to display information about the progress of the backup, so all you have now is the sytem preferences. Sad. Apple should fix both of these.

same issue - tried multiple solutions but ultimately had to uninstall the antivirus (Avira) - I read others fixed this the same way even if they had other antivirus software

I have two macs so on the air I tried the following with success. I deleted the TM backup, then set up TM as if it was a new computer, it is still backing up without problems. I have Sophos which may be slowing it but I didn't change any configurations. My question is: since it appears the problem is not being able to amend the present backup would renaming the computer so that you start a new backup without deleting the old work so that you can still access the older files and possibly rename again when Apple fixes this problem work?

As others have found, Time Machine backups apparently never get started after upgrading to Sierra 10.12.  When TM "preparing to backup" goes on indefinitely, I notice in the OS Console:

repeats indefinitely (with the time changing on each occurrence).

I have an Airport Extreme 6th generation with a 2 TB hard drive connected via USB to the Airport Extreme.  It has been working reliably for backing up 2 MB Pro's across my LAN, in El Capitan and Mavericks, and earlier OS's--that is, working reliably for several years.  I do have a vague recollection of having a similar OS upgrade issue with Time Machine a few years ago.  My solution then was to directly attach the HD via USB to each MBP, move the .sparsebundle files to another machine's external HD or just remove the backups, and create new backup files on the original HD when reconnected to the Airport Extreme again.  Of course, the backup history was lost in this process, but at least I had complete backups of each current MBP Pro's file systems after the OS upgrade.

As a temporary solution to the current Sierra issue, this past week I connected directly via USB an external drive separately for each MBP, which contained NO Time Machine backups for either MBP.  After uninstalling or disabling Sophos, I was able to successfully backup both Macs.  If Sophos was running, the backups were going to take weeks to complete (about 440 GB on one MBP and 110 GB on the other).  With Sophos disabled or absent, the backups were completed within hours.

So several things appear to be relevant to this Sierra problem:  backup across the LAN vs. to a directly attached external HD via USB, whether or not the .sparsebundle (Time Machine backup) file is brand new or being amended via Time Machine, and whether or not files being backed up are being scanned by Sophos in real time.

I have 2 Mac Mini's also running, each of which has a directly attached (USB 3) external HD for Time Machine backups (i.e., not attempting to backup each Mini across the LAN).  Both of the Mini's have made the upgrade to OS Sierra without issue, and Time Machine backups are amending the .sparsebundle files as before, with no need to disable Sophos.

I sure hope Apple fixes this Time Machine issue very quickly!  Having unattended automatic backups is absolutely essential, and must be rock solid.  Being able to restore a machine from Time Machine backups is a must.  I've been working with PCs, Macs and Unix systems for decades.  When something bad happens to a file system, the ability to restore it from a very recent backup is mission critical.  Come on Apple, without reliable backups is like driving a car with no brakes.  Please get this issue resolved ASAP!