Fixing Dual‑Monitor Issues on macOS with the Dell Pro Thunderbolt 4 Smart Dock (SD25TB4)

macoshardwarethunderboltdual-monitor

This guide documents the troubleshooting session I had while getting two independent external monitors working on macOS with the Dell Pro Thunderbolt 4 Smart Dock (SD25TB4). It covers the symptoms, root causes, firmware requirements, and the wiring configuration that worked for my setup.

1. The problem

I received an SD25TB4 dock and it worked immediately with my Windows machine. On macOS, the experience was very different: both monitors would light up, briefly show the desktop, and then disappear.

I started with the usual cable and port checks, but nothing held. In the past, I would usually settle for a one-monitor fallback on macOS, but this time I used an AI agent to push the troubleshooting further and narrow down what was actually failing.

I started with the prompt:

$ I connected my mac to two dell docking station and the monitors showed and then disappeared why.

What follows is a cleaned-up version of that agent-assisted analysis, edited to reflect what actually worked in my setup.

2. Overview

Many macOS users experience issues when connecting dual monitors through the SD25TB4 dock. Common symptoms include:

  • Displays flash on briefly, then lose signal
  • macOS detects both monitors as one combined ultra‑wide display
  • Only one monitor works at a time
  • Clamshell mode (lid closed) does not activate

These issues stem from how the dock routes video signals internally and how macOS handles MST (Multi‑Stream Transport).

3. Why Displays Flash On and Then Lose Signal

Initially, the dock may:

  1. Negotiate a Thunderbolt connection
  2. Attempt to initialize DisplayPort streams
  3. Fail the DP handshake
  4. Reset the video pipeline

This results in monitors turning on briefly and then going black.

Root Cause: Outdated Dock Firmware

The SD25TB4 requires firmware updates to work reliably with macOS. Without them, the DisplayPort hub inside the dock fails link training.

Fix: Update Firmware Using a Windows Machine

  • Install the Dell Dock Firmware Update Utility
  • Update all available dock components (Thunderbolt controller, PD controller, MST hub, USB hub, etc.). In my case, only part of the stack needed updates.

After updating, the displays stop flickering and remain stable.

Note: I use this Dell dock primarily with my Windows machine.

4. Why macOS Sees Both Monitors as One Display

After firmware updates, macOS may still show both monitors as one giant display. This happens when the dock outputs a tiled MST signal.

Important: macOS Does Not Support MST for Multiple Displays

  • Windows supports MST
  • macOS does not

If the dock outputs MST, macOS merges both monitors into one, which is exactly what I saw.

Key Insight

Even if your monitors (e.g., Dell P2720D) do not support MST, the dock can still output MST internally.

This is why macOS shows one combined display.

5. Understanding the Dock’s Internal Video Paths

The SD25TB4 has two different types of video paths:

A. MST Hub (macOS‑incompatible)

  • DisplayPort 1
  • DisplayPort 2
  • Rear USB‑C port

These ports share an internal MST hub. macOS cannot split MST, so it merges displays.

B. Direct Display Streams (macOS‑compatible)

  • HDMI port
  • Front Thunderbolt 4 port

These bypass the MST hub and provide independent display streams.

6. The Final Working Configuration

To get two independent extended displays on macOS, you must use ports that bypass the MST hub.

Working Setup

  • Monitor 1 → HDMI → Dock HDMI port
  • Monitor 2 → HDMI → USB‑C (front Thunderbolt 4 port) on the dock

Do Not Use

  • DisplayPort 1
  • DisplayPort 2
  • Rear USB‑C port

These ports route through the MST hub and cause macOS to merge displays.

7. Resetting the Dock’s MST Hub

After rewiring, I performed a full reset sequence. I cannot confirm this step is strictly required, but I did it multiple times during testing and included it here because it may help stabilize detection.

  1. Unplug all monitors
  2. Unplug the dock’s power cable
  3. Wait 20–30 seconds
  4. Plug the dock power back in
  5. Connect HDMI monitor
  6. Connect USB‑C → HDMI/DP monitor
  7. Connect the Mac last

This forces the dock to re‑enumerate displays correctly.

8. Enabling Extended Display Mode on macOS

Once macOS detects both monitors separately:

  1. Open System Settings → Displays
  2. Select each monitor
  3. Set Use As → Extended Display
  4. Arrange displays as desired

9. Using Clamshell Mode (Lid Closed)

With two external displays working:

  • Keep the Mac connected to power
  • Use an external keyboard/mouse
  • Close the lid

macOS will switch to external‑only mode.

10. Summary

The Dell Pro Thunderbolt 4 Smart Dock can work with macOS when using the correct ports, but my setup is not fully reliable yet. If both monitors go into standby, macOS sometimes wakes with only one display detected, and I need to reconnect the dock to restore both monitors.

Key Takeaways

  • Firmware updates are essential
  • macOS cannot split MST signals
  • The dock’s DisplayPort ports use MST → avoid them
  • HDMI + front Thunderbolt 4 port provide independent streams
  • This configuration enables dual extended displays and clamshell mode

With the correct wiring and a fully updated dock, macOS can support two external monitors plus the laptop display, even if occasional reconnects are still needed in my case.

11. Final Working Wiring Diagram

[Monitor 1 HDMI] ─── HDMI Cable ─── [Dock HDMI]

[Monitor 2 HDMI] ─── HDMI Cable ─── [USB‑C → HDMI Adapter] ─── [Dock Front Thunderbolt 4 Port]

This configuration bypasses MST entirely and ensures macOS sees each monitor independently.