Need help on transfering files

Basically I’m going to be getting a new hard drive and make my current hard drive a slave, but I would like to keep some of the files on this drive i have now.

is it possible to just install windows on my new drive, take my current one (with windows in it still) and put it as a slave, boot my computer up and select which drive to run windows on and then just transfer files that way? or can that not happen at all?

You can do that, just like the way you say you will do.

So I can access my slave drive with windows installed on it while on my main drive
and just drag n drop files between hard drives without any risk of some sort?

Yep.