how to move storage to a different disk

Catalina was two Volumes on the Internal Drive.

The Mac HD - Data volume is where your files and apps are kept and you have access to them just like the older system volumes.

The Macintosh HD volume is where the system and system support files are kept and the user has no access to them. It's part of the new security that Catalina brings to the table.

Attempting to move ( If even possible ) the " User storage " runs a very high Risk of making the Computer cease to Function - period.

You aren't clear by what you mean when you say "User storage". If you mean various files such as photos or movies, then you can simply drag the icons over to create copies on the external disk. Then they can be erased on the computer's hard drive. If you are talking about moving the so called "Other", that cannot be changed. It sounds as it you may be wanting to move that "other" to an external disk with another OS on it. If so, it can't b done. Here is some information about the "Other".

The Other Category should not be altered, modified, move or deleted since it is part of the Mac’s operating system. Doing so would corrupt the OS and render the OS inoperative requiring a Wipe and Re-Installation.

The Other category is a potpourri of files which include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files

• Fonts, plugins, extensions

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

They can be located anywhere on your hard drive. The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.

I hope this helps you. If not, then please be a bit more specific about your question.

Additional information on similar question here https://discussions.apple.com/thread/253592029

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