Your system information

• Steam client version (build number or date):
• Distribution (e.g. Ubuntu):
• Opted into Steam client beta?: [Yes/No]
• Have you checked for system updates?: [Yes/No]
• Steam Logs: [generate by running this command in a terminal tar -zcvf ~/Desktop/steam-logs.tar.gz ~/.steam/steam/logs]
• GPU: Nvidia/Intel/AMD

Please describe your issue in as much detail as possible:

Describe what you expected should happen and what did happen. Please link any large code pastes as a Github Gist

Steps for reproducing this issue:

1. Not possible adding a external drive.
2. Success with manual editing library folders.vdf
3. But marking this drive as default is not possible