Thanks for the advice - these steps worked for me after changing a drive letter:
- Rename your existing Unreal Engine directory to something like UE_4.26-Temp
- Start a new Install of Unreal Engine via the Epic Games Launcher
- Pause the installation once the new download has started
- Move all the files from within the Temporary folder to the newly created installation directory - you can simply skip any files that show a permissions error (because the directory is ‘currently in use’)
- Then simply resume the download in the Epic Games Launcher and it will begin verifying all the files