Apple - How can I add new folders to the Favorites in the Finder sidebar?

Drag the folder to the desired position in the sidebar. This adds the folder to the sidebar in every current and future Finder window, as well as Finder Open/Save windows/sheets.

Removal is done by dragging the folder from the sidebar out to the side.

To enhance the accepted answer, paraphrase another, correct one more and to add my own input, I offer the below summary:

1. Drag and Drop

   Dragging and dropping the folder use to work for me, but seems to be largely hit and miss these days. For me I'm usually adding network folders and these sometimes don't drag too well. If it does, and the horizontal bar appears in Favourites where you would like, then well and good. If not, try the following options:

2. Keyboard Shortcut: CTRL+CMD+T

   The Old keyboard shortcut use to be CMD + T but that got dropped when Finder introduced tabs. That now opens a new tab. The keyboard shortcut has become CTRL+CMD+T.

3. Finder "File" menu

   There is an option on the finder menu to "Add to Sidebar" (and you can see the keyboard shortcut listed to the right)

When the above don't work

Often I find that I can't drag and drop the folder to the sidebar and this can be frustrating, and going to the File menu sees the "Add to sidebar" as disabled or greyed out. But if you use the shortcut in this scenario, it should still work (it has worked for me just now under El Capitan).

Also, the "hack" within these answers of "drag and drop a subfolder and press escape" trick has also allowed me to add it. (Thanks @LWTBP, I didn't know this method before tonight).

For me, the scenario was:

• I had a network share I wanted to add to Favourites
• I'd brought up the particular folder in the navigation area of Finder but every time I dragged it to Favourites, it would only allow me to Drop it onto an existing folder
• I opened the carat to a subfolder and attempted to drag that in and the horizontal bar appeared
• I pressed ESC, re-selected the parent folder and dropped it in the desired Favourites position
• And all was right with the world :)

Sometime a folder might not show the "horizontal location-line" graphic as you drag it into the sidebar, making it impossible to attach it to the sidebar. In such situations, you can drag a folder within the folder into the sidebar and cancel that action (press 'esc', etc.) once you see the location line graphic. Then try dragging the main folder again onto the sidebar again—it will work.