Wampserver is not a basic portable application. Running Wamp64 after copying to a USB U: requires more modifications. Supposed the original install is J:\wamp64 :
Define INSTALL_DIR u:/wamp64 in u:\wamp64\bin\apache\apache2.4.33\conf\httpd.conf (only affects internal Wampserver scripts)
Replace all j:/wamp64 by u:/wamp64 in files : 1. U:\wamp64\bin\apache\apache2.4.33\conf\extra\httpd.vhosts.conf
Before lauching U:\wamp64\wampmanager.exe "as an administrator" we must delete old services with opening a command windows and type : sc delete wampapache64 sc delete wampmysqld64 sc delete wampmariadb64
Now we can launch U:\wamp64\wampmanager.exe "as an administrator", the tray icon will be red. We have to create new services by Left1.Click and
It works, I did it exactly like this, on a U:\wamp64 USB stick after copying my original j:\wamp64 folder.
Important : The USB stick must be NTFS formatted and always take the letter U: at each connection.