Can 2012 Mac mini disk be shared on macOS Catalina?

Open the Sharing pane of System Preferences and enable File Sharing.

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You can share the drive over the network or a specific folder if that's what you mean.

click here ➜ Set up file sharing on macOS Catalina - Apple Support

You can't determine a specific amount to be shared though.

Mu-iOS wrote:

I have a 2012 Mac mini with 1TB of SSD, I am looking to share its disk with both my MacBook Pro and MacBook Air where both have little storage (256gb), I am aware that (maybe?) sharing a disk requires macOS Big Sur 11, but is it not possible to do it on Catalina?

Were you thinking about Target Disk Mode, where you start up an Intel-based Mac in a special mode where it acts like a directly-attached drive for another Mac?

That used to work over FireWire, maybe over USB as well - but these days, there's a restriction. If either computer is running Big Sur or later, the connection between the computers must be a Thunderbolt one.

Network file sharing - where both computers are up and running, but one can access some of the files on another as if it were a NAS - is a different thing. I haven't heard of any restriction saying that network file sharing requires Big Sur or later. Of course, if you do use network file sharing, there are things you should not do – such as moving the Photo libraries from your MacBook Pro and MacBook Air to the Mac mini's (NAS's) drive.

If you are planning to use the Mini as a network share (the easiest), pretty much any Mac can be made into a network share/file server.

Personally, I am using a 2010 MacMini running High Sierra as a file server and file archive. I simply set up what ever drives or folders that I want to share across my network on the Mini in the File Sharing preferences.

Using a Mini in Target Disk Mode just to use its drive drive is a pain and wastes power compared to just having a stand alone external drive. FWIW, if you want to use the drive, it is not all that difficult to remove the drive from a 2012 Mini and get a cheap USB 3 enclosure for it and then recycle the rest of the Mini.

Perhaps a better solution might be go get an external SSD portable drive for each device like this one:

OWC Envoy Pro MINI or if budget is a factor this:

It to is very portable and easy to carry. I have 4 of them as external SSDs and Time Machine.

They can be used even when the device is away from the home WiFi network.