Windows license and external harddrive

Before purchasing a large-capacity external SATA or USB harddrive (such
as 1 TB) and using it as a system drive, I want to know what will happen
to Windows (e.g. XP) license because I intend to move around the system
drive between more than one computers.
(I understand that MS collects hardware information from the computer in
which a Windows is installed and that license is tied to that particular
computer hardware architecture. Since an "external" harddrive is meant
to be moved from one computer to another, is it supposed to be used as a
storage only?)