Start laptop on external hard drive with Windows 10?

I took an old hard drive out of another laptop (can boot normally, is windows 8 / 64bit on it) and want to boot over my laptop, because I accidentally installed a 32bit version on it ages ago, but a 64bit based system. So and because my options are too complicated and it takes too long to install a 64bit version again, I wanted to start from my other bootable hard drive. (It's easy according to the internet) So follow all the steps. F2 Bootmenu and put my external hard drive on the top position so that it has the highest priority. Only it starts every time via the built-in hard drive and I have no idea why, since it has Windows 8 bootable on it normally and also runs. Any tips? Do I have to set anything in the menu, on the hard drive?

Thanks for the answers, I'm sorry for grammatical errors and a nice Monday morning!

What do you have against a 32 bit system?

What are you doing for mega things that you need a 64 win on an old LT? And then also Win 8? It's silly.

Because with a 64bit system only a part is used and 2GB usable RAM of 8GB is actually stupid. Just want him to run better because he has the opportunity. Of course, I don't want to do anything elaborate on it anymore.

The limiting factor could be the hardware.