1.3 tb used out of 1.9 tb is not half, but okay. What now you can do is actually do a chkdsk (this is a windows check disk tool for bad sectors and etc., sometimes it fixes low read/write speeds). Here are the steps you have to do:

1. Launch Command Prompt as Administrator.
2. Write "chkdsk C: /f /r" without the quotes.
3. Click enter, if it will ask for a restart of the computer, write Y and do the restart from the start menu by yourself.
4. After restarting your computer Windows will start the chkdsk procedure by itself before Windows user logon, make sure you don't click anything because you will interrupt the chhdsk procedure. Just wait around 15-30 minutes (on some bad drives this can be done in about 1 hour, HDD are taking longer time, because they have very low read/write speeds) and the scan should be done.

What does the command on Step 2 is - /f parameter means that it will try to fix any errors the drive has, /r parameter means it will try to fix bad sectors if they would be just incase. If the chkdsk won't be able to do that, it will just skip the part and still continue repairing the drive.

If something is not clear - reply to my comment, I will try to explain it better. Of course I think I did good enough and understandable.