hdd gets assigned same device letter
 

So, Uhm. I guess because I'm not sure how to exactly describe it, I can't find anything on google. I have 5 hdds in my machine. 1 has the OS and the other four are 2 raid 1 arrays. I add in another hdd and it breaks one of my raid arrays. It becomes the new /dev/sde and bumps one of my raid array drives to /dev/sdf. How do I stop it from doing this?

Usually by mounting the devices by the UUID. I don't know much about raid arrays, but you should be able to set it up to use the disk UUID (either via /etc/fstab or udev or some other raid specific command/program).

so how does one go about doing that?