Below is a complete, copy-paste-ready fix for the exact error you’re seeing in Arduino IDE 2.x:

Unknown FQBN: missing platform arduino:hexbright referenced by board hardware:hexbright:hexbright.
Could not connect to /dev/cu.usbserial-0001 serial port.

Root cause:

  1. The IDE can’t find the HexBright platform package (it expects a full “core” install, not just boards.txt).
  2. The port name /dev/cu.usbserial-0001 is not the CDC-ACM device the HexBright uses.

ONE-TIME FIX (takes < 2 minutes)

1. Install the HexBright core via Boards Manager URL

  1. Open Arduino IDE → File → Preferences.

  2. In “Additional Boards Manager URLs”, paste exactly this line (if there are others, separate with commas):

    <https://raw.githubusercontent.com/dhiltonp/arduino-boards/master/package_hexbright_index.json>

  3. Click OK.

2. Install the core

  1. Tools → Board → Boards Manager…
  2. Search: hexbright
  3. Install “HexBright by David Hilton” (version 1.0.0 or newer). → This downloads the full platform (arduino:hexbright) and fixes the FQBN error.

3. Fix the wrong serial port

The HexBright appears as a CDC-ACM device, not usbserial.

After installing the core:

  1. Unplug & replug the HexBright.

  2. Tools → Port → you should now see something like:

    /dev/cu.usbmodemXXXX1   (HexBright)

    (The exact number changes; pick the one with “HexBright” in parentheses.)

    If you still see only /dev/cu.usbserial-0001:

    → That’s a different USB-serial adapter (e.g., a CP2102 or CH340). The HexBright is not on that port. Unplug everything else and re-plug only the HexBright.