Ziloz :
Will a computer start to slow down as you fill a hdd? and would adding another 1TB hdd do anything to help it?
Hard drives don't slow down when filled like solid state drives do. What does slow down read/write speeds is fragmentation and the position of the files on the hard drive. Fragmentation is when a file is split into multiple parts on the hard drive. The like a record player, the hard drive head will take time to get from one side of the disc to the other just to read one file. Files located near the center of the disc are going to read / write quicker because the head naturally resets to that position when not in operation and the length of the track is shortest near the center.
Adding another 1 TB hard drive will only help with your computer's speed if you plan to install the OS on it and only if it's faster than your current hard drive. If you want your OS to be very quick, just buy an SSD and install your OS on that. SSDs are much faster than hard drives.