An error occurred preparing the software update - Big Sur

I contacted Apple and got it to work. It’s because the computer doesn’t have enough space. So back up your laptop to an external hard drive and free up space.

to bypass the update and go back to regular log-in:

turn the computer off by holding down the power button

press and hold the option button BEFORE turning back on the computer

keep pressing option and turn the computer on

once the computer is on, you should be at the screen that says 2 options for Mac

select Mac HD

log into your wifi

now it should start up again and log back to Home Screen.

I was FINALLY able to resolve this, without wiping my hard drive and losing all my data. You need another computer running OSX 10.13 or 10.14. Here's the steps I followed (big thanks to https://mrmacintosh.com/big-sur-upgrade-not-enough-hd-space-serious-issue-possible-data-loss/)

1. Plug problem laptop into desktop with thunderbolt cable, laptop in Target Disk Mode
2. Desktop is running 10.15 Catalina. In Disk Utility, I see "Macintosh HD - Data - Data" which is the folder I need to access, but when I click Mount and enter my password, it is rejected. This is a problem with Catalina..
3. Download OSX 10.14 Mojave
4. Using the third party software DiskDrill I turned Flashdrive 1 into a bootable install disk of Mojave.
5. Restarted Desktop holding Option key. Clicked install Mojave. Installed on 64GB Flashdrive 2.
6. Booted from flashdrive 2.
7. Opened Disk Utility, Laptop in Target Disk Mode, mounted Macintosh HD - Data - Data. Password works now!
8. Deleted a bunch of files until I had over 30 GB free.
9. Restarted Laptop, install finishes successfully!

I had the same exact error: I tried installing Big Sur with about ~15GBish but apparently you need ~35GB to actually install it. After the error "an error occured preparing this update" appeared --> i shut it down and turned it back on --> just goes back into the normal login page --> mac tries to install Big Sur again and fails --> "an error occured preparing this update" --> endless loop

I fixed it after spending close to a week working through a bunch of trial and errors!

Here's what I've tried that DIDN'T work:

• internet recovery
• "normal" recovery: my password wasn't accepted even though it's 100% correct and the option to recover via the apple iCloud wasn't successful (tells me that my iCloud account can't be used for my mac...)
• resetting the SMC
• resetting the NVRAM

Here's what I did that WORKED (at least for me):

1. Started up this mac in the Target Disk Mode (shut down the mac and press T to go into this mode)
2. Connect this mac to another mac with the thunderbolt cable
3. Start the other mac (i.e. working mac) in recovery mode --> go into disk utility --> attempt to unencrpyt the hard drive of the mac that's having trouble (I can't remember if my password was accepted then but I don't think so)
4. Start your working mac in a normal manner --> login --> you will be prompted to type the password for the hard drive of the mac that's having trouble. They accepted my password finally (and any prompts from here onwards accepts my correct password)! Now delete some stuff in the hard drive of the mac that's working! I deleted some applications that are easily downloadable from the internet/are free until my drive had about 58GB of free space (no specific reason why I freed 58GB - I just wanted to play it safe). Eject the hard drive.
5. Start up the mac that's not working in recovery mode! The recovery mode should now be the "normal" recovery menu with the 4 options (use Safari, choose disk utility, time machine, reinstall Big Sur). I chose the reinstall Big Sur option and it redownloaded BIg Sur and then installed it later. AND IT WORKED. I WAS ABLE TO LOGIN INTO MY MAC AND SEE ALL MY ORIGINAL FILES BACK PHEW.

Let me know if you need any clarification. I'm more than happy to help because I know how it feels to be stuck troubleshooting for what seems like infinity and be anxious about losing all of your files. Ping me if necessary!

I called Apple support and they erased my Macintosh HD drive completely, used my Time Machine Backup to get my Mac to its previous state (Mojave) and then i downloaded the Big Sur again and installed it without problems. It took me about 35 min to install it.

They told me that probably there was a problem with the first downloaded file and it caused many problems to the structure of my hard drive (at some point I head 3 Macintosh HD showing up in Disk utility). If you have a TM backup, go ahead and erase everything do everything from the start. Good luck everyone.

After few attempts I manage to access my data (running on Mojave before trying to upgrade) without erasing the disk. I did not have my mac backed up on an external driver, so here the steps I took:

– access recovery mode by pressing shift+cmd+alt+r at the same time before clicking the power button. You will see a world graphic spinning. Insert your internet password and after about 15mins you will see a prompt with different options.

• select the last option ‘disk utility’
• under the folder ‘internal’, you will see the disk ‘Update’. I tried erasing but it did not work, so I unmounted it by selecting it and clicking on ‘unmount’. That disk will be light grey now. You could see that I only had 15gb free so it is probably what caused the issue.
• Now I run the disk ‘Macintosh HD’ by selecting and clicking on the run option via the top left menu. Be aware i had another disk greyed out ‘macintosh HD-data’, which I haven’t touched. A prompt appears where you have to ‘unlock’ the disk. You will ask to insert your password (this time it works!). The system will reboot and voilà. All my files were there!
• note: before trying to run the ‘’macintosh HD’ try to ‘first-aid’ and see if it allows you to unlock it. I always had errors, so i try running it again.

I will now make sure to backup and free some space before attempting the upgrade.

Hope this helps!

Hey there,

I had also this very annoying issue mentioned here.

I got almost insane cause I tried every suggestion.

At the end, one small hint in a video of mrmacintosh got my attention: https://youtu.be/bpXm_V_IJhc?t=85

The DNS settings of my router.

I am using PiHole and nextDNS. So I turned those off and used the default DNS of my internet provider. Afterwards I downloaded Big Sur again, created a USB installer, used the USB installer to erase my SSD again (just to be sure) and finally got the installing running without any issues.

Hopefully this advice will also help other people.

I would never have thought that my DNS server could be to blame.

Alright so I managed to get it to work without losing any data. Here are my steps.

Get an external drive. (SSD) is preferred.

Power on Mac then hold Shift, Options, Command, and R. This should bring you to a screen with a spinning globe and will ask for the wifi password.

Plug your external drive into the Mac and go to disk utility.

Click on view on top left and click show all devices

Click on your external drive and click erase. Select the following options before clicking erase again: Whatever you want to name your drive, APFS Format, GUID Scheme.

Click Erase

Afterwards go back to the macOS utilities page and select Reinstall macOS.

Accept the terms and conditions and then select the external drive as a boot device to download the OS to.

Then just wait for it to finish and then boot up following the instructions indicated.

Once in, go to Finder and then select Macintosh HD Data and navigate to user and then just open your desktop or documents folders to access your files.

Copy everything to another external device like a USB stick if the files are small in size.

Then remove at least 15gb of space remaining and turn off your laptop

Boot on your laptop again and hold the same key combination as mentioned before: Shift, Options, Command, R.

This time reinstall macOS to the Macintosh HD drive. There should be enough storage to do this. Otherwise delete more files.

Then the OS should be installed on your laptop and your external drive is no longer needed.

You now have your laptop back and your files saved.

Good Luck.

I recently had this issue, and found the solution from this website (all credit to Mr. Macintosh for solving my problem!):

https://mrmacintosh.com/big-sur-upgrade-not-enough-hd-space-serious-issue-possible-data-loss/

My issue was that the Big Sur update didn't check whether there was enough free space on the hard drive before trying to install. So you need to free up some space for the update to complete. Here is the extract to retain your data:

13. How to fix this issue + retain your data!

This section is divided into 2 parts. Part one is for if your Mac is encrypted with FileVault 2. The 2nd section is for if your Mac is not encrypted.

FileVault 2 Enabled

When FileVault is enabled on macOS Catalina or Big Sur, you are required to enter your account password before entering macOS recovery. This is critical because the only way to recover (retain user data) a FileVault enabled Mac will be with the help of a 2nd Mac.

If your Mac is encrypted and you need to recover your data, you have 2 options. One with just your mac and a USB Drive and the other with a 2nd Mac and a Thunderbolt or USB Cable using Target Disk Mode.

Option #1 – Install macOS Mojave on a external USB, SD Card or Hard drive.

1. You can boot from this external version of macOS. This will allow you to clear enough space from “Macintosh HD – Data”
2. Use an external USB flash drive or external hard drive. You install macOS on an external USB, SDCard or hard drive. Once Mojave is installed from recovery to the external drive boot to it by holding down Option after reboot. Once booted to mojave, you will see your drives on the desktop. You can then mount “Macintosh HD – Data” from Disk Utility. Now you can delete files from the “Macintosh HD – Data” then reboot. The Big Sur upgrade will finish.

Option #2 – 2nd Mac Required – Target Disk Mode

1. You need a 2nd Mac running macOS High Sierra or Mojave. (Big Sur & Catalina Mac fix instructions coming soon)
2. One of the following cables are required. USB-C to USB-C 3.0 Cable, or a ThunderBolt 3 Cable.
3. Once you have the target Mac in Target Disk Mode, you can use the host mac running 10.14 or 10.13 to mount the disk after entering your encryption password. > https://support.apple.com/guide/mac-help/transfer-files-mac-computers-target-disk-mode-mchlp1443/mac
4. As soon as you plug in your cable from the target mac to the host mac, you will get a pop-up window asking you to enter the password for your problem Mac.

Hi LizethBo,

What you need to do is boot off of a USB stick of MacOS to try to restore from back up to Catalina if you can. You might need to use a second Mac to create the USB stick.

Once restored, you need to create a new container in the APFS volume for the installation. Download the installer again under Catalina and it should allow you to install on the other volume.

Also, see my comment you earlier for additional details.

Hope this helps.

Hello all, I had this same problem, and I was able to mostly recover my data, hopefully this is helpful for some of you!

What I did:

1. After I got the message that the install failed, I clicked the thing that said restart in targeted disk mode.
2. connected it to a second Mac with thunderbolt cable
3. For some reason, it wouldn’t accept my password when I try to mount the drive on the second computer. It also rejected the password in the recovery mode that others suggested.
4. Downloaded Disk Drill software. This scanned the drive and showed all my files! My password worked fine for this software (who knows why!). The free version let me see the files, but then I had to pay $97 for the “pro” version to recover the files. They’re mostly there. Some of the older files are broken or something, they won’t open. But better than nothing!
5. Restore computer from scratch. Haven’t done this yet, should be pretty straightforward, but who knows, it may be a nightmare!

P.S. Apple-why did you do this to me? My weekend was ruined and my wife won’t speak to me after I almost lost all of her files. All for a stupid os update, ugh.

Okay I had the same basic experience

I was running MacOS Mojave and it was running well no complaints but I don't like to get to far behind so I ran a time machine backup first unencrypted.

Then ran the update to MacOS Big Sur

I got the same error "An error occurred preparing the software update"

Tried recovery mode but that did not work as it would not unlock the drive with the correct password (this is troubling and makes me question file vault ) I also tried using my apple ID to unlock the drive but this did not work as it could not unlock the drive.

Tried Safemode but that would give me the dreaded Circle with a line in it

I figured out it was most likely due to lack of storage space. Then found this post.

This is what I did.

I used a MacOS Mojave usbkey installer (I made a while back)

I install MacOS Mojave to an external drive (Nvme SSD usb c drive)

after successfully booting to the external SSD drive

I then was able to unlock my internal drive (the password worked) and I remove 50 GB of data

Then shutdown disconnected the external SSD

Then rebooted to the internal drive and the install of Big Sur Finished

I know I should have looked at the requirements prior to upgrading.

I would hope in the future the Installer for all MacOS Updates checks to make sure the system meets the needed space requirements.

Hope that helps.

Brian

heya all

I also can confirm that instructions from the article can help you

https://mrmacintosh.com/big-sur-upgrade-not-enough-hd-space-serious-issue-possible-data-loss/

(or some scenarios in the comments)

we had broken update from Catalina to Big Sur on MacBook Pro 2015 and encrypted disk.

we connected to another Mac (with Big Sur) in target disk mode, couldn't mount the data volume because of the "wrong" password (probably with macOS High Sierra or Mojave it will work).

many thanks to Apple Support that they suggested to delete system volume and after that the data volume could be finally mounted and we've copied all necessary data (there is similar story in the comments of that article).

good luck to everyone

https://mrmacintosh.com/big-sur-upgrade-not-enough-hd-space-serious-issue-possible-data-loss/

https://youtu.be/Qcu9o4qz2Ls

This guy figured it out (Mr. McIntosh). What a lifesaver! He saved my wife’s computer. I thought we were going to have to wipe her hard drive. You just need the right cable to connect to an external hard drive or another MacBook and then you can erase some enough data to make room for the install. Apparently you need about 35 GB for the install to work but the Big Sur will somehow let you download and go through the install if you have less than that but more than I think about 10 GB. So you somehow get stuck in this loop. I had to get an old thunderbolt cable which isn’t readily available at most electronic stores to connect my two MacBooks. His video goes through all the different scenarios that one might encounter. So helpful. Apple should pay him for finding a solution to this.

Tracy,

Thanks for the two updates.

Resetting SMC and PRAM may not show a visible effects but they do help in a small way. Good to know your disk is in good shape.

Regarding the key combinations, you are correct that one is able to reinstall the OS that came with the machine. There is also one to upgrade to the latest version that is compatible with your Mac which is option command r

I wouldn't bother calling Apple or stopping by the genius bar as that will take time and probably go over some of the same steps. Just my thoughts.

Do you happen to have FileVault enabled or a firmware password set?

Glad you were able to start up in Internet Recovery, that's good news. Since you were able to start up in Internet recovery you could try reinstalling the operating system this way, I assume it's Mojave. If you can find your USB of Mojave you could try that as it will save the download time, I don't know how fast your internet connection is. If you can't find your Mojave USB then use Internet Recovery.

Hope you are able to reinstall MacOS Mojave successfully and then upgrade to Big Sur afterwards. If somehow you aren't, there is one last step we can try but will share that later as I've wrote a lot already.

Hope this helps and let me know how it goes.

Hi Tracy,

I appreciate you getting back to me with the update.

That's good to hear you were able to do a fresh Mojave install and that you erased the internal drive. Very interesting about the mystery though.

One thing to consider would be to boot off the Big Sur USB installer you created. Hold down option when you boot your Mac, select Big Sur from the boot selector.

Next go to Disk Utility from the MacOS Utilities screen and wipe the drive again. Quit disk utility and select Install MacOS from the MacOS Utilities screen and see if that works.

Crossing both my fingers and toes that this works.

Hope this helps and keep me posted.