I need answers

[DrZander]

I am building my first rig and was curious if i could take the os (windows 7,8) from an old computer and transfer it to the new rig. Also, if that isnt possible then would it be better to just install windows 10 on an external hard drive and then i will always have it for future rigs? Is this how os's work?

[Juanse]

No, you should always make a fresh install on a new build, if not, errors and instabilty might accur

[KraftDinner]

You could still use the OS with a fresh install, all you need is the key when they prompt you to enter it.

[failblox]

If you have the product key from the version of windows on your old computer, then yes, you should be able to. If not, then I don't think it's possible.
 

Installing Windows on an external HDD for future rigs could technically work, but it would be a very bad idea. Theoretically you could already do this by moving the internal HDD from your old computer to your new computer, but Windows doesn't like it sometimes when it finds out the computer it's running on has different hardware from before and it might deactivate itself. Furthermore, drivers vary from computer to computer, again, because of different hardware, so you won't be able to have a "one size fits all" Windows installation. Also, running Windows off an external HDD would be slower than running Windows off an internal HDD.

 

TL;DR: It's not practical and you'll run into lots of issues. Just do a fresh install.