Big Sur Installation problem: "An error occurred preparing the software update" (no space issue)

PROBLEM ##: Mac OS X Big Sur Installation failed because of lower disk space. Your disk is encrypted. So,
a) Recovery mode (hold Cmd + R while booting) not working: Not accepting your password to let you in.
b) Target disk mode (hold T while booting) not working: You connected your mac to a second mac as an external storage using appropriate cables/adapters, but in the second laptop you do not see the encrypted disk as an external disk, neither get any prompt to enter the encryption key anywhere.
c) Internet recovery mode (hold Cmd + Option + R while booting) not working: Internet recovery is attempting to install Big Sur only, but not the original OS X version with which your mac initially came.
SOLUTION ##: The solution is to somehow open the macOS Utilities at boot. Recovery mode can do that but as you cannot enter it, you have to trick the system to open macOS Utilities. What worked for me was this - make a bootable installer for any previous macOS version in a USB flash drive following the instructions here support.apple.com/en-gb/HT201372. You will not install it really (so any version will work), but this is just for letting the mac read this USB drive as a bootable source (this is the trick). Once you do this, plugin the USB drive and then do the following:
a) Boot while holding the Option key. This is used to boot from any specified device and will show list of all devices.
b) In the device list, you will see the USB Flash drive. Select it and go next. Don’t worry, it will not install anything before asking you.
c) In some seconds, you will see the macOS Utilities open (see pic below). Our purpose is served.
d) Click Utilities - Terminal on the menu (on top). From Terminal, using the following command line codes you can first unlock the encrypted disk using your password (worked for me), then delete some files to increase storage.
1) See all volumes in the mac (see pic):
diskutil apfs list
2) Identify the volume (disk) that is locked with filevault encryption and note its volume id (disk1s1 in the pic).
3) Unlock the volume with the following command and entering your password when prompted.
diskutil apfs unlockVolume /dev/volume_id
4) Once unlocked, you can see the encrypted volume within /Volumes/
5) Delete anything you want to make storage.
[If necessary you can also decrypt the volume using instructions here derflounder.wordpress.com/201...]
e) Then, you can restart mac and install Big Sur normally.

[@hrachyastepanyan3680]

Thanks a lot.
The video helped me. I used the terminal "rm -r file/directory" command to clean up free space for installation.

[@sccr]

Hey did it actually help?
I have a problem. I installed Big Sur and then my computer crashed. I followed the rules of command, options, shift +R and it took me to utility disk and I did the first aid and it gave me error. My mac is full 250gb with no available space. What can I do? Please someone tell me they had the same problem and they solved it?

[@nilotpalsanyal]

@@sccr Can you open macOS Utilities by any way? If yes, then you can open terminal from there and follow the above instructions to delete some files to make space. Msg me personally if you cannot work it out and I will try to help.

[@swoosh714]

@@nilotpalsanyal hello, I was able to unlock, but how do I delete files now?

[@nilotpalsanyal]

@@swoosh714 Find your data disk inside 'cd /Volumes/'. Go to files/directories you want to delete. Delete a file with 'rm filename'. Delete a directory with 'rm -r directoryname'.

[@swoosh714]

@@nilotpalsanyal sorry, but how do you find your data disk? Am I supposed to still be in “terminal”? How do I search for the files and directories? Sorry for all the questions. Not very tech savvy. Thanks

[@Tket93]

Thank you very much for the video. I am now at the stage after i have input the command “diskutil apfs unlockVolume /dev/volume_id”. It is now asking me to input my passphrase which i assumed is my password to gain access to my mac. However, it doesn’t seem to be working. Your advice would be much appreciated.

[@nematix]

Hey, thanks for the video. This is the closest I've got to fixing it but still not quite there. I got into terminal and deleted files to make some space, but when I reboot afterwards I'm still stuck in a loop and I can't get it to resume the big sur installation. I get my login screen, enter my password, get a progress bar for a few seconds then it reboots to the apple logo, progress bar under the apple logo for a few seconds, then back to the login screen. Any ideas?

[@joonasnik]

I am ecstatic! Sounds like this could work for me. Have to try it today.

[@joonasnik]

This is the magical chain to rescue one’s data