There are several reasons why just copying and pasting the drive contents into an external and then trying to run it will fail.
Chief is you would be missing the boot record. secondary, but almost as important, the change in hardware would be so massive from one PC to another, that either A) The machine does not boot and nothing works, and you get blue screens of death for un-found drivers, for the hardware to such an extent the machine is unusable. Or B) the machine does boot, but Windows detects the changes in hardware, knows its a different machine and deactivates itself, requiring reactivation.
You would also need to install new drivers for all the new hardware.
And Third. Its not a simple thing to get a machine to boot from and external source, unless the BIOS allows this, and even then it needs to be properly installed there, not just copied and pasted.
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.