Golden Gate. Unable to add folders from data volume to sidebar.

To add items to the Finder Sidebar, you can simply drag and drop them there. You will see the insertion marker (a line with a circle at the left side) that moves up and down the list as you move the item, release it where you want the item to be positioned.

Note that you can only add custom items to the Favorites section of the Sidebar. You cannot add anything to the Locations section, that is controlled only by the checkboxes in Finder menu > Settings > Sidebar tab.

Note that if you cannot see the Favorites section, it may be collapsed. Hover your cursor at the right edge of the Sidebar next to Favorites and an expansion caret (>) will appear. Click that to expand the section.

More info: Customise the Finder sidebar on Mac - Apple Support

I would recommend some basic troubleshooting steps, since dragging and dropping a folder into the Sidebar is working fine for me.

I'm not sure what you mean by 'data volume'. The above folder was dragged there from my internal drive. But I am also able to add a folder to the sidebar by drag-and-drop from an external SSD:

...and from a volume on a network drive / NAS:

Have you rebooted your Mac? If not, that's the first step. If that does not help, then see if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.

Start up your Mac in safe mode - Apple Support

If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.

Add a user or group on Mac - Apple Support

Once the testing is complete, the new account can easily be deleted.

Delete a user or group on Mac – Apple Support

Those steps will help to isolate the problem.

dmarroni wrote:

Many thanks for the reply. Drag and drop is no longer working, but only for folders in the data volume.

Where are these folders actually located? The "Data" volume is too broad a term since there are a lot of hidden items located on the "Data" volume, but hidden & located outside of your home user folder.

If these are folders contained within your home user folder, then which top level folder in your home user folder holds these troublesome folders?

I'd also suspect an issue with third party software that may be interfering with the normal operation of macOS. The usual culprits are anti-virus software, cleaning & optimizer apps, and third party security software.....none of which are needed on macOS. With a Finder issue, another possibility is some sort of third party add-on Finder extension. You can try booting into Safe Mode to see if that makes any difference.

You can also try creating a new macOS user account. Log out of your main user account, then log into the new user account. If this works, then you have some sort of Login Item, Finder extension, or configuration setting issue with the main user account.

dmarroni wrote:

Golden Gate. Unable to add folders from data volume to sidebar.

How long is your list of items in the Sidebar?

The one situation where I couldn't drag a folder (Documents, as it was), it happened that it was already there, except that I had to scroll the list to see it.

On a different note:

What exactly do you mean by the "data volume"?

We know that macOS uses a separate system volume and a data volume, but in Finder these are not seen as separate entities.

If you show us an example of a particular folder that you cannot drag, it may be become clearer.

Many thanks for the reply. Drag and drop is no longer working, but only for folders in the data volume.

Thanks again for the suggestions. Tried them all. No difference in safe mode. Can't drag and drop with the new user.