User profile for user: Apple-Marc
Apple-Marc Author
User level: Level 1
8 points

Time Machine stuck on "Preparing backup", then fails.

Whenever I try to make a Time Machine backup, Time Machine takes between 1 and 2 hours "Preparing backup", and then fails with the message "Time Machine couldn't complete the backup. An error occurred while preparing the backup."


The backup was started over a year ago and there was never a problem, up until a few days ago. It's also not corrupt, and I can enter Time Machine and browse the backup. There's also around 450GB of free space on the backup drive.


I'm running Big Sur 11.2.3 (20D91) on an early 2015 MacBook Pro, and I'm trying to back up the contents of the internal 512GB SSD onto an external 4TB USB drive.


What I've tried:

  • I've deleted the .inProgress file and tried again. Failed.
  • I've ran First Aid on both my Mac's drive, as well as the external backup drive. Multiple times without any issues, on all existing partitions. All drives seem in good health. Backup keeps failing.
  • I've disabled automatic Time Machine backup, removed my drive from Time Machine, rebooted, re-added the drive in Time Machine... failed again.
  • I've removed my drive from within the Time Machine GUI and inherited the backup via tmutil (sudo tmutil inheritbackup machine_directory), which is basically the same measure described in the previous bullet point. That seemed to have worked fine - but the next backup attempt took again failed after 2 hours of "Preparing backup".
  • I've disabled Spotlight with "csrutil disable". Backup keeps failing.
  • I've booted into Recovery mode from USB, and ran First Aid for all drives (internal & external) from there. No issues - all drives seem in good health.
  • I've listed all partitions with "diskutil apfs list" and ran "fsck_apfs" on all disks/partitions. Nothing suspicious. All good.
  • I've monitored the backupd process with the console, and now all my hope is that someone can pinpoint the real issue here, or explain what this error 303 means. I've done this over 5 times now (1-2 hours per attempt), and the log always has the same error code 303. Here is (part of) the console log from the time of failing of the backup:
error	04:05:47.960875+0200	backupd	zero fileid for  (00000003)
error	04:05:48.004160+0200	backupd	zero fileid for  (00000003)
error	04:05:48.292272+0200	backupd	zero fileid for  (00000003)
error	04:06:53.488248+0200	backupd	Event collection timing out because no FSEvent callbacks received in more than 40 seconds
error	04:07:02.644180+0200	backupd	Failed to collect events for "SSD - Data" (device: /dev/disk1s1 mount: '/System/Volumes/Data' fsUUID: FD624480-CA7E-4D85-8E3F-F1519FB8B95A eventDBUUID: 071AE26C-45E8-4950-9321-67800EF904EA), error Error Domain=com.apple.backupd.ErrorDomain Code=303 "(null)"
default	04:07:08.116447+0200	backupd	Received configuration update from daemon (initial)
error	04:07:08.455054+0200	backupd	Backup failed (303: BACKUP_FAILED_PREFLIGHT_STAGE_GATHER_EVENTS)
default	04:07:08.455411+0200	backupd	MessageTracer: Falling back to default whitelist
default	04:07:08.873096+0200	powerd	Process backupd.276 TurnedOff BackgroundTask "Time Machine backup" age:01:55:39  id:47244673101 [System: PrevIdle DeclUser kDisp]
default	04:07:08.891889+0200	powerd	Process backupd.276 TurnedOff PreventUserIdleSystemSleep "Time Machine backup" age:01:55:39  id:4295000143 [System: PrevIdle DeclUser kDisp]
default	04:07:09.018682+0200	tccd	AUTHREQ_ATTRIBUTION: msgID=2887.1, attribution={requesting={identifier=com.apple.TMHelperAgent, pid=2887, auid=501, euid=501, binary_path=/System/Library/CoreServices/backupd.bundle/Contents/Resources/TMHelperAgent.app/Contents/MacOS/TMHelperAgent}, },
default	04:07:09.032968+0200	tccd	-[TCCDAccessIdentity staticCode]: static code for: identifier com.apple.TMHelperAgent, type: 0: 0x7fabb4748410 at /System/Library/CoreServices/backupd.bundle/Contents/Resources/TMHelperAgent.app
default	04:07:09.083132+0200	tccd	AUTHREQ_ATTRIBUTION: msgID=139.133, attribution={accessing={identifier=com.apple.TMHelperAgent, pid=2887, auid=501, euid=501, binary_path=/System/Library/CoreServices/backupd.bundle/Contents/Resources/TMHelperAgent.app/Contents/MacOS/TMHelperAgent}, requesting={identifier=com.apple.WindowServer, pid=139, auid=88, euid=88, binary_path=/System/Library/PrivateFrameworks/SkyLight.framework/Versions/A/Resources/WindowServer}, },
default	04:07:09.083288+0200	tccd	requestor: identifier=com.apple.WindowServer, pid=139, auid=88, euid=88, binary_path=/System/Library/PrivateFrameworks/SkyLight.framework/Versions/A/Resources/WindowServer is checking access for accessor identifier=com.apple.TMHelperAgent, pid=2887, auid=501, euid=501, binary_path=/System/Library/CoreServices/backupd.bundle/Contents/Resources/TMHelperAgent.app/Contents/MacOS/TMHelperAgent
default	04:07:09.092919+0200	tccd	-[TCCDAccessIdentity staticCode]: static code for: identifier com.apple.TMHelperAgent, type: 0: 0x7fabb4748410 at /System/Library/CoreServices/backupd.bundle/Contents/Resources/TMHelperAgent.app

MacBook Pro 13″, macOS 11.2

Posted on Apr 12, 2021 8:12 PM

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5 replies
User profile for user: DiZoE
DiZoE
User level: Level 4
3,033 points

Apr 13, 2021 10:11 PM in response to Apple-Marc

Hey there!


I’ve seen numerous causes with that error, I don’t believe it’s a tell-all type of error message.


Have you happened to try backing up while booted to safe mode? This may point to a cause by third party software installed, (Obviously the file system seems okay as you’ve run fsck, etc).


Likewise, can you click “Options” in Time Machine Preferences, and click the option to exclude apps and system files and try again? This may also reveal a possible cause.


I’ve, in the past had to exclude several different directories to finally find a possible culprit.


Likewise, are there other users that are logged into your Mac when you backup?


You can also run “tmutil status” in Terminal for more information, although I’ve found if it’s in the “preparing” stage, it usually doesn’t progress.


Likewise, is either your Mac and/or external drive encrypted?


Also, and I know this isn’t easy to just test, does the same external drive backup other Macs just fine? Likewise, does your Mac backup to other external drives just fine?


Cheers. Apple Care may also be able to help you possibly pin point a cause, but it may take time to do so.


I wouldn’t condone the software offered in this article that I found, however some of the steps they provide make sense:


https://fixmacissue.com/fix-error-code-303-on-mac.html


Hope something helps, let me know some of those answers and will answer best I can thanks.

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User profile for user: DiZoE
DiZoE
User level: Level 4
3,033 points

Apr 14, 2021 9:38 PM in response to Apple-Marc

Hey again! No worries, glad to help out as I’ve been there as well.


You could even, (And again this is for testing purposes), boot your Mac to recovery mode, (Startup holding Command R, (Or holding power depending)), in Disk Utility, add an APFS Volume on your HD, reinstall the OS, set up as new and backup to test it out on a fresh partition, then test it out, it’s a big step, I know, but sometimes you gotta do what you gotta do.


I have an archive of “OS X” (Yes the old OS’) error codes, I’m no programmer or engineer, so not sure if this helps, but the 303 error shows up here:




In searching this particular error, found a few more resources searching:


SmRevisionErr 303, etc. mostly insecure websites and old information, but did find a few with useful information on it. Not the best advice, but it is what it is.


Best of luck, will help further if I can cheers.

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User profile for user: Apple-Marc
Apple-Marc Author
User level: Level 1
8 points

Apr 13, 2021 5:12 PM in response to MoonJ.

Well yes, I'm receiving an error when trying to complete the backup – and very fittingly, it says "Time Machine couldn't complete the backup. An error occurred while preparing the backup." Unfortunately that does not help me with diagnosing WHY the backup fails during or right after "preparing".


I was really hoping that someone could shed some light onto what this error 303 means. Anyone?!


I (obviously) had already tried and checked the different things listed on If you can't back up or restore your Mac using Time Machine. Pressing and holding the Option key to verify a backup has never been an option for me though – the "Verify Backups" option has always been greyed out, since the beginning of the backup. (Backing up from an APFS SSD onto an encrypted HFS+ external HDD.)


In addition to everything I listed in my post, I have now also deleted /Library/Preferences/com.apple.TimeMachine.plist (multiple times), and that doesn't bring any change either.


I always delete the .inProgress file, run First Aid multiple times, reboot, delete the Time Machine preferences file, start over the backup inheriting, always monitoring backupd with the console – I keep trying various combinations and orders of the above ... I'm now easily at 15 failed backup attempts of 1.5-2 hours each.

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User profile for user: Apple-Marc
Apple-Marc Author
User level: Level 1
8 points

Apr 14, 2021 4:02 PM in response to DiZoE

Thank you very much @DiZoE for your longer reply and many (new) recommendations!


I'll try to go over everything you have mentioned:


  1. Yes, today I did everything I had listed before and then ran/attempted a Time Machine backup in safe mode. That too failed, with the same log entries as all previous attempts.
  2. I have excluded /Library and /Applications, but that didn't make any difference so far.
  3. There are no other users logged in. Never.
  4. I have not run "tmutil status" yet, but I will do that next time. (Although if you say there isn't much to be found out in the "Preparing" stage, then I might not see anything at all.)
  5. My Mac is not encrypted, but the external backup drive is encrypted (Mac OS Extended, journaled, encrypted).
  6. Since I don't own (or have access to) a 2nd Mac device, I indeed can't simply try to backup a different system to the same backup drive. But I guess I can attempt a fresh backup on an empty old spare drive, without deleting the current 3.5TB backup.
  7. Yeah. I'd love to get some (remote) assistance directly from Apple in this case. It's been a highly frustrating experience so far.
  8. I already landed on that fixmacissue.com website myself, after trying to find out any infos on that error 303 that I'm always seeing in my log files. But that website seems to just mention a general "Error 303"; one that isn't specific to Time Machine. One thing I had not tried yet, was resetting the PRAM. So I did that – before running an attempt in safe mode – but that didn't change anything.


Thanks for your help so far!

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Time Machine stuck on "Preparing backup", then fails.

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