Can't access "Macintosh HD" on networked computers

Is the Macintosh HD drive shared on these machines? It's not shared by default.

To check, open System Preferences on the target Mac and from there, the Sharing pane. There will be a check in the box to the left of "File Sharing" if you can access other drives but you also have to give specific access rights. Select File Sharing and look at the list of Shared Folders to the right of it. If Macintosh HD is not in the list you'll need to add it by clicking the "+" under the list, browsing to the drive and clicking the "add" button. Once added (or if it's already there) select it and check the access permissions in the Users list.

I'd not normally attempt to share the full hard drive and didn't test it before posting. I've just shared the drive on one of my Macs and, when attempting to access the share, I ran into the same problem. I haven't seen anything to suggest this was an intentional change by Apple but other than separately accessing each subfolder of the root directory I don't know if there is any way around the problem.

on Catalina macOS 10.15.7 I had the same problem as you described.

In the Sharing section of the preferences pane I added my home folder in the shared folders section. I made sure the "Users" section of my Home folder included Administrators and my User Name and both had read & write ability. I was then allowed to login on the network and have full access to the drive. I hope this works for you.

yes, I mean add your actual "home folder". Your right your Mac HD will already be in there.

in Sharing, in preferences folder, make sure "file sharing is checked and highlighted.

in Shared folders section you mac HD will be there but you will need to add your personal "home folder".

select your home folder in the "Shared folders" window and make sure Administrators and your name (home folder) are set to read and write.