Let’s say I was going to replace my old hard drive, how would I transfer UE4 over to new drive without completely re-downloading it? (Download would take several days), and I will no longer have the old HDD? Is there any way to transfer the installation rather than re-downloading? I am talking about the engine, not the launcher.
I think it should work when you just copy the 4.7 or 4.8 folder onto your new HDD - then re-install the launcher and move the 4.8 folder from your HDD into the unreal engine installation folder 
tried. didn’t work 
Although I read somewhere you have to copy a folder called ‘epic’ (I think) from “C:/ProgramData” too
unfortunately for me I read that AFTER I re-installed windows (and copied ONLY the installed folder, i.e, X:/Program Files/Epic Games/and all that stuff.