Even with the "top" methods, issues appear.
Problem: "Unity is already running" error on a new PC.
Fix: Delete U:\Unity\Cache\Lock file and U:\Projects\MyGame\Library\ArtifactDB. Unity locks artifacts by machine ID.
Problem: Android SDK or IL2CPP missing.
Fix: You must install Android Build Support, NDK, and JDK directly on the portable drive. In Hub -> Preferences -> External Tools, point Android SDK to U:\Unity\AndroidSDK.
Problem: The PC asks for Admin rights to run Unity.
Fix: You didn't use the symlink method correctly. If you try to install Unity directly to Program Files on the external drive, it demands admin. Keep Unity in a root folder like U:\Unity.
Unlike the Hub-driven download, a portable installation begins with the Unity Editor Installer (typically an executable ending in .exe on Windows or a .dmg package on macOS, available via the Unity Download Archive).
The most cited method from top forum posts (Unity Discussions, Reddit r/Unity3D) involves:
Verdict: Not a true portable app, but a relocatable installation.