Q: MacOS Sierra - time machine stuck preparing backup on Network devices

I have the same problem. I found this,

macOS Sierra problems: Time Machine won't back up Mac

Time Machine problems have been preventing Mac owners from backing up their devices since upgrading to macOS Sierra, and the problem is widespread enough that Apple has released a guide on what to do if you can't back up:

SMC Reset for the iMac

1. Shut down the device

2. Disconnect the power cord from the monitor

3. Wait 60 seconds

4. Reconnect power cord

5. DO NOT POWER UP DEVICE

PRAM Reset

1. The computer should still be shut down.

2. Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.

3. Press and release the power button to turn on the computer.

4. Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears. Continue holding the keys down until the computer restarts, and you hear the startup sound for the 3rd time.

5. Release the keys.

You can then try backing up again. If that doesn't work, try turning off indexing on the drive you're backing up.

but i am not sure it is the same issue. I will try it and report back.

Had the same problem after the Sierra update...stuck at 1kb in the preparing backup mode - worked fine with an external drive.  After modifying the excluded list as you suggested...it sat at 5kb while preparing backup for about 5 minutes, then started and completed a proper backup again. Thanks a bunch.

Restart your Mac.

After I had this issue for nearly 4 hours I got it to finally back up from time machine I don't know if it will help others .. I moved the old back up from el captain probably work for any os . Transferred them files to a new partiton or place delete the old copy os el captain in my case, you can keep you old os in  different place... And then set it back up to back up the new os in the original place you normally back up.. For some reason the os x sierra gets confused with backing up with old existing os.

same problem here. tried smc / pram and spotlight exclusion - no joy. as a last resort i erased time capsule through airport utility and started fresh - was backing up successfully within minutes. no problems since.

I had the same issue. It turns out that my antivirus (Sophos Home) was causing an issue. I disabled this and my backup ran perfectly.

I was able to resolve my problem by going to Sophos Home website and turning off it's protection.

After the reboot 'Preparing backup...' took much less time and the actual transfer to my network drive finally started.

Thank you. This solution works for me. I deleted the old copy on Time Machine after the OS update, then the backup started.

My upgrade from El Capitan to macOS Sierra worked like charm in relation to TM backups - iMac continues to backup to 3TB TC over network with 2 routers.

If stuck on preparing a backup try connecting to you TC in Finder, then open TC in mounted devices and locate file that says in progress, restart MAC and TC and main router.

Hi

Just let everybody know, tried a couple of things, changing the exclusion list as suggested, did not seem to do the trick. I am running sophos Home, so turned that off and at first did not seem to work, then after about 3 or four hours i got a message saying i was already connected to the TM, I rebooted, fired of the backup again and now its working like a charm. Sophos home is still switched off, will run back up for a few days to make sure it is running constantly then switch sophos back on and see if that stops it again. If it does will bang on the doors of Sophos and see if they have a fix. Will let people know here

Remove Sophos and Trusteer, neither are necessary or desirable. Remove them per the manufacturers directions, don't just toss them in the trash.