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Upgrade from Catalina to Big Sur on my MacBook Air (11 inch, early 2014) fails - "There is not enough free space on the selected volume to upgrade the OS. An additional 28.37GB is required"
Note that the 512GB drive (APPLE SSD SM0512G) has 6.6GB available (it seems that this install requires 35GB!) I have an external 1TB Samsung SSD; can I use it for the install and how?
MacBook Air
Note that the 512GB drive (APPLE SSD SM0512G) has 6.6GB available (it seems that this install requires 35GB!) I have an external 1TB Samsung SSD; ...
Use that external SSD for a Time Machine backup, assuming you are not already doing that. Creating a backup is a fundamental prerequisite for upgrading macOS.
I understand the remaining internal storage is insufficient for the upgrade. First please review Free up storage space on your Mac - Apple Support and Optimize storage space on your Mac - Apple Support.
If you are still unable to create sufficient space for the upgrade after exhausting all those steps, the workaround is to create a Time Machine backup, completely erase the Mac, install the latest version of macOS (read • below), and finally transfer the contents from that Time Machine backup when asked. To learn how to use Time Machine please read Back up your files with Time Machine on Mac - Apple Support.
Rebuild the Spotlight index on your Mac
What is “Other” storage on a Mac, and how can I clean it out?
How to delete Time Machine snapshots on your Mac
See used and available storage space on your Mac
Locate backups of your iPhone, iPad, and iPod touch
Notation - If the user is using a cloning software like Carbon Copy Cloner - suggest tweaking the Safety Net Feature in this software. It may be making additional Snap Shots that are not being Cloned to the Eternal Drive. If this should be the case, these Snap Shot could be using additional space on the drive
The final word from Apple on Managing the " Other/ System Data “ Category
Other / System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.
FWIW, you don't have near enough available space on your Mac to run optimally, much less upgrade. 10% free is the bare minimum you should have.
How to upgrade from Catalina to Big Sur with insufficient storage available
