The better solution, at least for me, was to move your whole Umbrel install to your external drive. This is more resilient across upgrades to the Bitcoin app, and you won't run into any linking issues with apps that use the Bitcoin app as a dependency.
First, make sure your Umbrel instance is stopped:
cd ~/umbrel
sudo ./scripts/stop
Backup your Umbrel folder first (you can delete later):
sudo cp /home/umbrel/umbrel /home/umbrel/umbrel-DELETE -r
Move your install to your external drive:
sudo mv /home/umbrel/umbrel /media/umbrel/node/umbrel -r
Create a soft-link between the original filename and the link name - kind of like a shortcut in Windows:
sudo ln -s /media/umbrel/node/umbrel /home/umbrel/umbrel
That's it. Again - If you found this helpful, please consider an up-vote or send some sats to: guttermonk@getalby.com
Thank you. Stay happy. Zap sats.