Since updating to Catalina two months ago I have had this problem: My MacBook Pro HDMI output is attached to a NEC monitor. Every time I log in on my standard account the monitors are mirrored and Catalina only recognizes the MBP screen. There is no way to turn off mirroring or select the second screen-it doesn't show in system preferences, which only detects the MBP retina screen.
****About this Mac**** Chipset Model: Intel Iris Pro Type: GPU Bus: Built-In VRAM (Dynamic, Max): 1536 MB Vendor: Intel Device ID: 0x0d26 Revision ID: 0x0008 Metal: Supported, feature set macOS GPUFamily1 v4 Displays: Color LCD: Display Type: Built-In Retina LCD Resolution: 2880 x 1800 Retina Framebuffer Depth: 24-Bit Color (ARGB8888) Mirror: Off Online: Yes Automatically Adjust Brightness: No Connection Type: Internal **P221W:NEC Monitor ---** Resolution: 1680 x 1050 (Widescreen Super eXtended Graphics Array Plus) UI Looks like: 1680 x 1050 @ 60 Hz Framebuffer Depth: 24-Bit Color (ARGB8888) Display Serial Number: 07104621NA Main Display: Yes Mirror: Off Online: Yes Rotation: Supported Automatically Adjust Brightness: No
The problem didn't exist before the Catalina upgrade.