Can I boot an hdd without altering the original windows environment?

There's no guarantee Windows 10 will boot successfully if the installation wasn't performed on the same computer. It won't hurt anything on the internal drive to try it, though.

Unless the 2nd windows HD has a verifiable Serial Number for Windows that is tied to your computer it will boot it. You may have to use your computer's BIOS (Or whatever they call it) to recognize and boot from your external HD.

Features will be disabled if the S.N. do not match. Make sure you have all the drivers for your computer model on your external drive.

If you are paranoid, you can disable the internal in the BIOS.  I assume you want to try and boot from the external.

If you don't disable it, it will just look like a filesystem to the external boot and probably assign a drive letter and mount it.  You can use diskmgmt to unmount it to prevent scribbling on it if you don't disable it in BIOS.

If this is a disk that moved from one pc type to another, windows will try and update and convert the system.  So if you mean will the new disk be changed when you boot it, yes it will but only the OS for updates and changes for hardware.  If it has been updated and same hardware, I think it will not do the conversion process (probably).  And of course if you have any added hardware requiring drivers..