The issue is that your 1TB SATA hard drive's S.M.A.R.T. service is predicting that the drive will soon fail. The whole idea behind this S.M.A.R.T. service is so that drives can provide a warning that they are going to fail before they actually do fail. This gives you a chance to get a replacement drive and copy the data from the failing drive to the new drive before it is lost. For more information on S.M.A.R.T., start here: https://en.wikipedia.org/wiki/S.M.A.R.T..
BTW, the drive becoming slow is a further indication that it is having problems doing its job. In my mind, makes its replacement far more urgent. Do it now before you lose your data!!
Hope this helps,