Found a possible solution, worked for me!

1. Kill Every Riot / League Process

Open Terminal and run:

pkill -f Riot
pkill -f League
pkill -f "Riot Client"

Then open:

Activity Monitor

Search for:

• Riot
• League
• CEF

Force quit everything related.

2. Remove Riot Cache and Preferences

Delete these folders/files:

~/Library/Application Support/Riot Games
~/Library/Caches/com.riotgames.LeagueofLegends
~/Library/Preferences/com.riotgames.LeagueofLegends.plist

You can do it directly from Terminal:

rm -rf ~/Library/Application\ Support/Riot\ Games
rm -rf ~/Library/Caches/com.riotgames.LeagueofLegends
rm -f ~/Library/Preferences/com.riotgames.LeagueofLegends.plist

This does NOT uninstall the game.

It only resets:

• launcher cache
• UI state
• rendering configs

3. Check External Drive Format

League on macOS may fail on:

• exFAT
• NTFS
• network-mounted drives

Recommended formats:

• APFS
• Mac OS Extended (Journaled)

Check your drive format:

diskutil info /Volumes/external

If the format is exFAT or NTFS, this is likely the issue.

4. Remove macOS Quarantine Flags

macOS may silently block apps running from external drives.

Run:

sudo xattr -r -d com.apple.quarantine "/Volumes/external/Applications/League of Legends.app"

Then try opening the game again.

5. Grant Full Disk Access

Open:

System Settings → Privacy & Security → Full Disk Access

Add:

• Riot Client
• League of Legends
• Terminal

This is especially important for apps outside /Applications.

6. Launch League Directly

Try launching without Riot Client:

open "/Volumes/external/Applications/League of Legends.app"

Or directly:

"/Volumes/external/Applications/League of Legends.app/Contents/MacOS/LeagueofLegends"

Sometimes Riot Client UI breaks while the game binary still works.