Basic Virus location question

[Haoming]

Are viruses only on the harddrive and do they stay on the harddrive?

My old pc is dead & I want to make sure that there's no problem with the hd. I am sure that there are a couple minor viruses on that old pc which I do not want on this pc. I want to use this pc to test out that old hd though & also need to backup some files to an external drive.
If I just take out my current pc's hd & temporary replace it with the old hd, can this pc get infected/harmed in anyway?

Can the external drive get infected from viruses on that old hd?

When I'm done swapping & testing that old hd, I'll swap my usual hd back in

Thanks!

[yosa]

Viruses can affect other drives, they can also target memory chips that are used by components, you will likley not have that problem as in my experience it is uncommon; so as you like with the spare drive, any aberant cp behavior is a red flag.