Catalina: Purpose of /System/Volumes/Data/Previous Content/?

Question

After the update from Mojave to Catalina I noticed that the used disk space has increased from about 1TB to almost 2TB. And there is hardly any free disk space left.

A closer inspection of the file structure of the whole system shows that about half of the data is in the folder /System/Volumes/Data/Previous Content/Users/myusername. It seems to be a mirror of my user data that was created during the upgrade. This would explain the sudden doubling of the disk space used.

I can't find any clear information about the purpose of the folder /System/Volumes/Data/Previous Content/Users/ neither here in the forum nor on the web. Is it possible that after upgrading and setting up all user data from the backup, the contents of the folder /System/Volumes/Data/Previous Content/Users/ can be deleted without problems? Since it does not seem to be necessary for the operation of Catalina?

(Note: this question is not about the general purpose of the APFS partition /System/Volumes/Data/ of the new APFS)

Answer 1

I think it was originally used for Archive and Install macOS installs. However, you haven't been able to select a Archive and Install for ages.

To me it looks like it is continuously updated in real-time which makes me think its actually linked to the original sources and the files you see are the actual files not copies. It also seems to be well hidden from the file system. Perhaps left there for comparability reasons.

Perhaps look at your drive using DaisyDisk to find where the space is getting used. macOS allows purge-able space take up to 80% of the drive, however, it should automatically reclaim when the storage is required.

https://daisydiskapp.com/manual/4/en/Topics/PurgeableSpace.html?source=PurgeableSpace&appEdition=Standard&appVersion=4.10&osVersion=10.15.5

Answer 2

For me, I had to first turn off System Integrity Protection which comes with Catalina onwards by rebooting using Command+R and executing this command from the terminal:

$ csrutil disable

Then I rebooted and was able to delete this folder, which in my case was taking up to 700+ GB of disk space:

/System/Volumes/Data/Previous Content

At this point, I was able to delete the above folder which you may need to execute as sudo (see https://discussions.apple.com/thread/250630022):

1. Open Terminal, which is located in the Utilities folder inside the Applications folder.
2. Type rm -rf (don't press Enter / Return just yet)
3. Add a space, then drag the folder into the Terminal window. Hit Enter (Return).

Then after of course I rebooted into the recovery screen using Command+R and re-enabled the system protection with the terminal:

$ csrutil enable

At this point your purgeable content should increase so though you may not immediately see an increase in free disk space the OS will manage this for you if you continue to install more Apps or add files so that you will have the disk space you need until the drive is full.