I can't open catalog from external HD

Hmm, a question that should have been asked.

What MACOS version?  (early MACO Ventura had an issue)

What LrC Version?

In MACOS, look at that drives properties, it might be set up as removable, as such LrC treats it like a SD card. (bad) In the properties, you should be able to fix that.

There seems to be a problem with the storage being misrecognized, whether it's on the OS side or on the Lightroom side.
It recognizes HD and SSD as devices like SD cards.
Catalog files cannot be stored on devices like SD cards.

If there is an OS update, try installing it.
If you have another HD, try that.
Even if there is a DCIM folder in the root of the HD, misrecognition may occur.

"a message appears saying that Lightroom catalogs cannot be opened on network volumes, removable memories or read-only volumes."

Verify with Finder and Texedit that you can create, delete, and modify text files on the volume (i.e. that it is in fact writeable). 

If it's not writeable, in Finder right-click the volume and do Get Info.  What does it show for Format?  If it says NTFS, then the issue is that Mac OS by itself cannot write NTFS volumes -- you'll need a third-party NTFS driver. Alternatively, reformat the volume as ExFAT and Mac will be able to write it.

If the volume is already writeable, then the problem is elsewhere and we can add more troubleshooting steps.